Well, you can believe that God had something to do with it but the Big Bang is the reason everything exists, you can believe that God caused the Big Bang. But just read up on the origin of Earth and the Big Bang and you will have your answer
Every religion has its own version of the fairy tale. Just pick whichever one you like, and decide to believe it. Or, you can study science for a more realistic answer. A better question is, what existed in the moments BEFORE the Big Bang? That's where you'll find God.
God spoke it into existance. In the book of Genesis it says, "and God said let there be light... and it was so" See. He spoke everything into existance. He had a plan first though. Then He spoke, and it happened just as He spoke.