Talking to mom???

Okay so I want to get on birth control. I'm almost 15. I've talked to her about it before but never went through with it. I'm not really sure how to make sure she does. I am sexually active. Which she does not know. Which I would tell her I have been if she asked. Can anyone te me how to bring it up? Maybe even ask if she'd take me to a free clinic to do if.