For all of its simplicity, this is a difficult question. We are so much lower than God and He is so much higher than we are. We know Him mostly by what He does and by certain characteristics which are usually called attributes. However, let me try to give a basic definition.
God is the personal being who created the world, who maintains the existence of all things, and who will bring all things to their final end. He is the self-existent One. That is, He exists of His own power without being created or coming into existence. He is eternal, being before, after, and outside of time. There is no limit to His power. Neither is there any limit to His knowledge or wisdom. He is the Supreme Ruler of all things. He is perfect in all His being.
Morally, God is all good. He is pure and holy with no uncleanness in any regard. He is true and has nothing false about Him. On one hand, He is all righteous and demands justice. On the other hand, He is pure love and is ready to show mercy and forgive. He is the perfect mixture of all qualities of goodness.
Though infinitely exalted above man, God made man with the qualities whereby he could have communion with God. Though angry with man's sin, He is grieved at the separation caused by sin. His demand for justice is so great that He cannot let one sin go unpunished, but His love for man is so great that He sent Jesus Christ to pay the sin debt for those who will trust in Jesus as Saviour.
Though this is only an introduction to the person of God, I hope that it helps you begin to get some idea of the greatness and goodness of God. The desire of the saints through the ages has been to know God. I hope you will seek to know Him as well.