First Exam

I'm fifteen and I'm about to get my first pelvic exam and Pap smear. My periods have been absolutely horrible. My mom finally agreed to getting me on the pill to regulate my period. Though I want to be on birth control. Which I know there is a pill for both. How can I talk to my mom about getting that kind? I have a sex life, but she doesn't know. I'd rather keep it that way.