Defining who and what God is difficult because God is--by nature--completely different from what we are. Therefore, we can only describe Him by analogy and cite His attribute as He has revealed to us as found in the Bible. Nevertheless, the Christian God is the only supreme, eternally self-aware being, who had no beginning nor will He cease to exist, who is non-contingent, transcendent, immutable, and of whom no greater being exists. His attributes include holiness, omniscience, omnisapience, omnipotence, omnipresence, logic, righteousness, justice, mercy, grace, etc. He is revealed to us in the person of Jesus as is described in the New Testament Scriptures.
God is the creator of all things (Isaiah 44:24). He alone is God (Isaiah 45:21, 22, 46:9, 47:8). There have never been any gods before Him nor will there be any after Him (Isaiah 43:10, 44:6, 8). God is God from all eternity (Psalm 90:2). In Exodus 3:14, God revealed His name to His people. The name commonly known in English is Jehovah. This comes from the four Hebrew consonants that spell the name of God. (See Tetragrammaton).
God is a Trinity, knows all things (1 John 3:20), can do all things (Jer. 32:17, 27--except those things against His nature like lie, break His word, cheat, steal, etc.,) and is everywhere all the time (Psalm 119:7-12).