That is a BIG question and the most important one you can ask. However I cannot answer it fully here, so I will give a short answer an then point you at some other information.
In many people’s minds God is the name that give to the person or thing that they know is “out there”, the one who is bigger than them and is in control of things that are beyond their own control. In this sense just about everyone has a concept of God.
The question, though, is what is this God like and can I relate to them. The only way we can know the answer to this question is if we could see and study this God or if God chooses to reveal to us who he is and what he is like.
Well the God of the universe has done just that. He wants to be known by us and so has revealed himself to us. He has done this in many ways. Firstly we can see evidence of God in the world around us but this doesn’t tell us much about what he is really like. So the second and more important way he has revealed himself is by coming down to us and living among us. This is who Jesus is.
Jesus is both God and a human being and so when we look at Jesus and read about what he did and said then we see God and can get an understanding of what God is like. All that Jesus did and said has been written down for us and is available for us in the bible.
So the short answer to you question - what (or who) is God - can be found in looking at Jesus Christ. So I encourage you to get hold of a bible and start reading one of the books that was written about Jesus - say Mark’s gospel - ans see what you find.
Jesus did more, however, than just simply show us what God is like. He also came to show us how to have a true and lasting relationship with God and this is also part of Jesus story.
For more information about God and Jesus and what he did to save us have a look at God Makes Sense on this website and get back to me if you have any other questions.