Nobody can adequately introduce Allah, but I would like to quote one of the greatest personalities in Islam, the son in law of Prophet Muhammad PBUH and the fourth caliph of Islam, Imam Ali ibn Abi Talib who is well-known for his eloquence and knowledge. This is how he described God:

“Praise be to Allah.
He is such that senses cannot perceive Him,
place cannot contain Him,
eyes cannot see Him and veils cannot cover Him.
He proves His eternity by the coming into existence of His creation,
and (also) by originating His creation (He proves) His existence,
and by their (mutual) similarity He proves that there is nothing similar to Him.
He is true in His promise. He is too high to be unjust to His creatures.
He stands by equity among His creation and practices justice over them in His commands.

He is One, but not by counting.
He is everlasting without any limit.
He is existent without any support. Minds admit of Him without (any activity of the) senses. Things which can be seen stand witness to Him without confronting Him.
Imagination cannot encompass Him.
He is not great in the sense that His volume is vast and so His body is also great.
Nor is He mighty in the sense that His limits should extend to the utmost and so His frame be extensive. But He is great in position and mighty in authority.”

He who assigns to Him (different) conditions does not believe in His Oneness,
nor does he who likens Him (to something) grasp His reality.
He who illustrates Him does not signify Him.
He who points at Him and imagines Him does not mean Him.
Everything that is known through itself has been created, and everything that exists by virtue of other things is the effect (of a cause).
He works but not with the help of instruments. He fixes measures but not with the activity of thinking. He is rich but not by acquisition.

Times do not keep company with Him, and implements do not help Him.
His Being precedes times.
His Existence precedes non-existence and His eternity precedes beginning.
By His creating the senses it is known that He has no senses.
By the contraries in various matters it is known that He has no contrary,
and by the similarity between things it is known that there is nothing similar to Him.
He has made light the contrary of darkness, brightness that of gloom,
dryness that of moisture and heat that of cold.

He produces affection among inimical things.
He fuses together diverse things, brings near remote things and separates things which are joined together.
He is not confined by limits, nor counted by numbers.

Stillness and motion do not occur in Him, and how can that thing occur in Him which He has Himself made to occur,