how should I go about asking my mom to go on birth control? (im 18 y/o!)

I am so ready to have sex with my boyfriend but I want to be completely safe so I want to be on the pill in addition to using a condom. The problem is i have to talk to my mom to get on the pill and I'm not sure how to go about it. She has asked me once before about a year ago and I said no but now I want to and idk how to ask! Pls help! Any advice will help!