How do you tell someone you're athiest?

For starters when I was little my family and I went to church every week. As I got older I did more things involving church like youth groups and camps. My mom and I stopped going to church long before that though. Now we started to go to church again and I find it boring and uninteresting. I'm a freshman and lately I've been thinking very highly of history and science. If you put it all together Christianity was not the first religion. So, anyway my point is, how do I tell my mom that I don 't fully believe in God and the Bible? I'm scared to tell her.