God, the Exalted, is the absolute being and perfection. There is no deficiency or imperfection in Him. He is unique and has the power to do everything. He has knowledge of everything at all times and places. He is All-Hearing and "Seeing". He has will power and free choice; He is alive and the Creator of all things. He is the source of all good. He loves all creatures and is Kind towards all of them.
The term “God” is one of the most general and simplest concepts that everyone, even atheists that deny His existence, definitely have some kind of perception of, although comprehending the true nature of his essence is out of our reach.
The ways to knowing and comprehending God can be summed up in the following ways:
1. Reason and intellect, like the argument of the necessary and possible being.
2. Through experimentation and sensation, such as the argument from design.[i]
3. The way of the heart or the argument of fitrah (genesis, innate disposition towards virtue and knowledge)[ii],[iii]
The easiest and the best way to understand and know God is the argument of fitrah whereby a person refers to his own God-gifted nature as well as to his inward where he sees God without any rational argument or any experimentation. Thus he reaches God through the way of the heart.
[i] - Just because this way utilizes experimentation doesn’t mean it is devoid of any reasoning and rationality, what is meant when it is said that it is experimental is that one of the premises of the argument is such, which is to observe the different phenomena of this world.
[ii] - Ma’arefe Eslami, vol. 1, pg. 41
[iii] - For further information see: Understanding God; Adopted from Question 479 (website:520.
In order to clarify our answer to the above question, it is necessary to take a few points into account:
1. The concept of “God” is one of the simplest concepts that even atheists can understand and comprehend, because everybody knows that God refers to the being that has created everything, that exists in this world, is capable of doing whatsoever he desires, is aware of everything that occurs and is all perceiving, the ever listening and etc.
2. Although the term "God" is one of the most general concepts, comprehending the true nature of his essence is out of our reach. The reason being that ‘knowledge’ means and happens when the ‘knower’ has total awareness of ‘the known’ subject, and due to the fact that man’s being and attributes are in almost every way finite, which results in a limited and finite knowledge, comprehending God’s infinite true essence is out of his reach (the limited cannot have total awareness of the unlimited because of the lack of capacity for such). “They cannot encompass God in knowledge” hence one should refrain from pondering God’s essence. Perhaps, it is because of the same reason that the Holy Quran while mentioning the attributes of God describes Him as Needless, Knowing, Wise, Seeing, Hearing, Exalted, Great, Compassionate, Merciful, Creator, Originator etc. 
In addition, man establishes communication with external objects which are the signs of God and which help mankind gain knowledge about the Lord. In fact, God, the Exalted is not like anything. "Nothing is like Him". «لیس کمثله شی»
Despite the fact that we cannot comprehend Him completely, everyone is capable of studying the divine attributes and understand them depending on the capacity he possesses.
The ways to knowing and comprehending God can be summed up in the following ways:
1. Reason, like the argument of necessity and contingency (the burhan of Wujub and Imkan)
2. Through experimentation and sensation, such as the argument from design
3. The inner way, the argument of fitrah (genesis, innate disposition towards virtue and knowledge)
4 - The easiest and the best way to understand and know God is the argument of fitrah (or the way of the heart) whereby a person refers to his own God-gifted nature and to his inward where he sees God without any rational argument or any experimental observation. There is always a luminous spot in man's heart which enables him to communicate with the metaphysical world. It is also the closest way to God.
5. Although knowing God, the tendency towards Him and the light of monotheism are ingrained in the nature of all human beings, the superstitious norms, customs, wrong educations, false inculcations, negligence, pride and a lot of other factors especially when one is healthy and enjoys all kinds of bounties, obstruct and overpower the God-gifted nature. But when he is surrounded by misery and hardship and when he despairs of worldly means and feels that he has no access to anything, it is only then that the curtains go away and that luminous spot becomes visible. It is only then that he drops all kinds of polytheistic thoughts and becomes purified in this oven of difficulties and mishaps turning to the metaphysical world. Man's turning to God during times of calamities is a sign of our inclinations to Godliness. Man gets involved with his position and status so much that he forgets about God. He gets fully entangled in his sins and disobedience. Once calamity befalls upon man, he suddenly remembers God and turns to Him for help. Thus, we realize that this sense of Godliness is intrinsic in humans, but man's involvement in worldly affairs results in his forgetting God.
That is why, there are different Quranic verses that remind mankind about the bounty of God-seeking nature.
The great leaders of Islam also guided in the same way those who went contemplating with hesitation about God. For example read the following piece of history:
A man went to see Imam Ja’far ibn Muhammad al-Sadiq (as) and expressed his amazement about knowing God.
The man said: "I have discussed this issue with many knowledgeable people, but they have not been able to help me." Imam Ja’far Sadiq (a.s) ended his amazement using his intrinsic human Godliness and asked him:
"Have you ever been aboard a ship?"
The man said: "Yes."
Then Imam Jafar Sadiq (a.s) asked: "Have you ever been on a sinking ship, and reached out for a narrow piece of board to stay afloat?"
The man said: "Yes. I have."
Then Imam Jafar Sadiq (a.s) said: "Then while you could not see any dry land or anyone there to save you, were you not really attracted to some power to rescue you?"
The man replied: "Yes."
Imam Sadiq (a.s) said: He is God Who has the power to save. There is no one else to save or to come a person's help."
The conclusion we can draw is that Godliness is intrinsic and that every individual has some tendency, perception and knowledge of God within his heart. If he is negligent of God due to some reasons, he cannot deny the incidents which have taken place in his lifetime and in which he has been bereaved of all means causing him to direct his attention to God.
6. Sometimes, man is led to God and His attributes such as knowledge, power and wisdom simply by observing and contemplating carefully over the qualities of things and their relationship with each other. Since this way is based on observation of the natural world and experimental study of natural phenomena, it is called the 'experimentation way'. Given the privileges of this way, the Holy Quran has attached special importance to it. Therefore, there are many verses calling on man to ponder over the worldly phenomena surrounding him because they serve as creational signs of God, the Lord of the universe. Some Muslim scholars, relying on certain aspects and features of the world of nature e.g. the design and pervasive order in natural things, have arranged an argument to prove God's existence. The argument is named "argument from design". Therefore, the argument from design can be named a clear example of what we just termed as 'knowing God through experimentation'.
Understanding through the ‘signs’ pointed to in the Quran and hadiths:
There are verses in different chapters of the Holy Quran which make mention of various natural phenomena considering them as signs and indications for God's existence and also calling on humans to contemplate over them. Knowing God through His creational signs in the world of creation which is a clear example of seeking God through the experimental way is sometimes interpreted as "understanding God through His signs". A number of verses call on mankind to contemplate and ponder over the creational signs of God. They describe the system existing in the universe and in man's being as a reason and guide that lead the wise and the thoughtful towards the origin of the universe:
"Most surely in the creation of the heavens and the earth and the alternation of the night and the day there are signs for men who understand."
And "On the earth are signs for those of assured Faith, As also in your own selves: Will ye not then see?"
There are numerous other verses in the Quran which have made reference to special phenomena and considered them as the signs of the existence, knowledge and power of God. The verses relating to this subject are so many that even mentioning few examples would need a great amount of space and time.
Religious leaders, following the Quranic method, have also emphasized on 'sign based understanding of God". For example, it has been reported that Imam Sadiq (a.s) said to one of his companions:
"O' Mufaddal, the structure of the universe is the foremost directive and argument for the existence of Almighty Allah -- how the parts thereof have been set together and been possessed of elegant workmanship and design. An appropriate mood of contemplation with reason focused on individual parts will disclose that this universe is comparable to a house furnished with all articles necessary for human beings.
The sky is like a canopy; the earth is spread like a carpet, while the stars set in stratum upon stratum, appear as lamps alight in their places. The gems are treasured as if the house has lots of collections. Besides these, everything is readily available to meet individual needs. Man, in this world, is like the masterful owner of the house, having in his possession everything therein.
And there exist the different plant species available for meeting; individual needs -- some as fodder for the animals, others as drugs for human beings; some merely for ornaments, some to supply fragrance to man for his recreation; some as drugs for animals, some as nutriment for man; some for birds only and others for the quadrupeds alone and so on. Different species of animals have been allotted functions for particular exigencies and interest.
These are reasons to prove that the universe has been created with wise and precise measuring, order and harmony. It is creator is One and He is the Designer and Originator of this harmony and order."
7. The rational way:
This is the way through which God is proved with purely rational preliminaries, principles and methods. 
The philosophical reasons and arguments for proving God’s existence are clear examples of rational investigation and research in line with proving God. Compared to the other two ways, this way has certain features of which we shall mention some as under:
1. Many of the rational arguments and explanations are not very useful for those who lack philosophical information because of the fact that philosophical arguments are complex and difficult in nature.
2. One of privileges of the rational way is that it can be used in our scientific debates with the atheists. Thus, we can reveal the weakness and flaw of their arguments by presenting rational arguments. The rational way can also be used to answer the challenges posed by rationalists and those who surrender not to anything but rational arguments.
3. The rational way used to prove the existence of God can be effective in strengthening religious beliefs because whenever a person’s mind submits to the truth, his heart also inclines towards it. On the other hand, rational argument plays a crucial role in removing doubts and hesitations and in curtailing any harm on one’s religious conviction and belief.
Taking into consideration the unique function of the intellectual method, and the mind’s natural inclination towards profound intellectual matters and discussions, Islamic scholars have made great efforts regarding understanding God through logic and intellect which have led to new arguments in this field, or the completion of previous ones. One of the most solid logic based reasons for proving the existence of God is the famous proof called “The Argument of Necessity and Possibility” (wujub wa imkan). There are different variations to this argument, but for our purposes we will use the one below. We can summarize this proof of necessity and possibility in this format: It is certain that there exists a being in the universe. If this being is the necessary existent, then our proof will be at hand. If it is a possible existent, then it has a need for a cause and since it is impossible to have an infinite chain of possible existents, this possible existent is in need of a necessary existent (whose existence is not bestowed by another cause). This existent is that of the Wajibul Wujud, or the necessary existent. To repeat this in other words, there are different kinds of possible existents in the world. They are either necessary existents or possible existents. The necessary existent does not derive its being from another cause or being, but the possible existent came into being due to another cause. All possible existents go back in a chain that must lead to a necessary existent, due to the fact that an infinite chain of existents is impossible.
Note: Since your question was precise and it was not clear whether your question dealt with God’s existence or with His attributes, we have endeavored to tackle the first possibility in greater length. In case, you are willing to know further about God’s attributes, you can email us again.
 - “لا یحیطون به علما” Taha:110.
 - Tafsir Namunah, vol.14, pg. 161.
 -Al-Shura, 11.
 - Just because this way utilizes experimentation doesn’t mean it is devoid of any reasoning and rationality, what is meant when it is said that it is experimental is that one of the premises of the argument is such, which is to observe the different phenomena of this world.
 - Ma'aref Islami, vol.1, pg. 41.
 - Tafsir Namunah, vol.16, pg. 341 & 342.
 - Ibid, pg. 418 – 422 & 423
 - Such as verse 65 Al-Ankabut and verse 30 of Sura Al-Room and verse 12, 22, and 23 of Sura Yunus, verse 67 and Sura al-Isra and verses 8 and 49 of Sura al-Zumar.
 - Behar al-Anwar, vol.3, pg. 41.
 - It should be noted that there are many viewpoints about "sign based understanding", a concept derived from the Holy Quran: Some researchers consider it as a prelude to the formation of a rational argument – similar to what was in "argument from design" – about God's existence, His knowledge and wisdom. However, based on another exegetic notion, the Quranic verses which call on man to contemplate over the natural phenomena are simply reminding him about his natural knowledge of God. The verses serve as reminders only. A third view is that the intended verses are aim to argue with the pagans, those who wrongly considered idols and their own hand-made statues as their lords and effective in the affairs of the world having no precise understanding of divine unity. (See: Tafsir Al-Mizan, Allamah Tabatabai, vol.18, pg. 154; Ideological Instruction, Mesbah Yazdi, vol.1-2, pg. 68, and Tabyin Barahin Ethbat, Javadi Amuli, pg. 43).
 - ان فی خلق السماوات والارض و اختلاف اللیل والنهار لآیات لاولی الالباب. [Aal-e Imran: 190]
 - و فی الارض آیات للموقنین. و فی انفسکم، افلا تبصرون. (Dhariyat, 20 & 21 and also see: Baqarah, 164; Jathiyah, 3 – 6; Yunus, 100 and 101; Ibrahim, 10).
 - Verses that emphasize on specific phenomena being signs are sorted out into few general categories. The verses relating to human life are: 1. Overall system of man's creation: Jathiya, 4, and Room,20. 2. Fetus formation system in the womb: Aal-e Imran, 6, Infitar, 6 – 7; Taghbarun, 3; Ghafir, 64, ; Hashr, 24; Noorh, 13 – 14. 3. Cognitive system: Nahl, 78. 4. Variety and multiplicity of languages and colors: Room, 22; Fatir, 27 and 28. 5. Sustenance and feeding system: Ghafir, 64; Isra, 70, Jathiyah 50 & 20; Fatir, 3, Room, 4; Saba, 24; Yunus, 31; Naml, 64, Mulk, 21, Anfal, 26; Baqarah, 22 & 172; Ibrahim, 22 and Dhariyat, 58. 6. Sleep system: Room, 23; Naml, 86; Furqan, 47; Naba, 9 and Zumar, 42. 7. Clothing system: A'raf, 26 and Nahl, 14 & 81. 8. Residential system: Nhajl, 80. 9. Marriage system: Room, 21; Shura, 11; Fatir, 11; Najm, 45; Qiyamat, 39; Nahjl, 72; Layl, 3; Naba, 8 and A'raf, 189.
 - Also, see: Nahjul Balaghah, sermon 186, Al-Tawhid, Shaykh Saduq, chapt. 2, hadtih 2, and Behar al-Anwar, Allamah Majlisi, vol.3, pp. 61, 82, 130, 152.
 - This way being called ‘rational’ does not necessarily mean that only reason or intellect is used to prove God. In fact, it means that it has sufficed to rational preliminaries and methods.
 - This meaning is not inconsistent with the rational way being considered of general and public utility because ‘being public’ is relative here as opposed to ‘being personal’ in the sense that the rational way is not particular to a single person. In fact, different people can benefit from it.
 - Rational is useful especially for those who are deprived of the capacity to see God through intuition and with the eye of the heart.