I started not believing in a god right before my 17th birthday. Your not too young to make a decision on religion. Just keep an open mind. Anyway, I don't believe in God for many different reasons. I've been reading books on both sides of the debate for 10 months now, and the arguments for God are fallacious, and the evidence for God is shaky and unreliable. I don't see any valid evidence for the existence of God. I recommend reading up on the subject yourself, if you haven't already.
Well, I'll answer this simply with a question:
Why do you not believe in Santa Claus?
It was because of lack of evidence. books that compiles all the hearsay about a divine being is not an evidence either.