Hi, I'm 15 and just became an atheist about 5-6 months ago. I've always been pretty skeptical, but always held my belief in God..... then I actually learned something :P
What really made me stop believing were the large amount of contradictions between religion and science. I guess I'm a bit of an oddity though, since I'm not a ex-Christian, I'm an ex-Jew. If you think Christian indoctrination is bad, you should visit a temple :)
I've come out to my friends and I don't really care who knows that I'm an atheist in my school, but I haven't found a way to tell my parents yet. The only other person in my family that knows is my sister, who is an agnostic. But my parents don't know about her either.
Anyway, it's good to find a community of people that think like me :D