Birth control please 😫

I'm about to be 18 and I'm a senior in High School and I've recently been in a serious relationship with someone I can see myself living the rest of my life with. We've recently had sex for the first time and it really brought us closer. I want to get on birth control because idk how far we can take it. But I don't know how to ask my mom without her knowing that I'm sexually active. So how should I do it?