The Pill?

Hello! I am planning on starting on birth control (pill). My doctor had talked about it for a bit with my mom and I (I am sixteen years old), but i'm a little nervous to bring it back up with my mom that i'd like to get on it. I happen to have very painful, long periods. Additionally, I told my boyfriend that we can not be sexually active until I am on the pill. One thing is that I don't know anything about it! Is there anything that I should know that my doctor won't mention?