Birth control

I don't know how to tell my mom I want to get on birth control. Me and my boyfriend are sexually active but she don't know that and really don't want her to know. It's anouther precaution I would like to have but I don't know what to tell my mom how I want to be on birth control. Dose anyone have any ideas on what to say??? (By the way I'm only 16 that's why I have to talk to her)