Birth Control

So I'm 17, will be 18 in a couple weeks, and I want to talk to my mom about getting on birth control. I'm currently dating this guy I've had a long back and forth history with, and we've been having sex but would feel more comfortable if I was on birth control. My parents have never been very accepting of anything serious that basically shows that I'm not a perfect, pure angel, but I'd rather go through the Hell that is talking to them about sex than end up pregnant because they've always made me feel like I can't talk to them. 
How do I start that conversation with them?