Asking for birth control

My boyfriend and I have been talking about sex lately and we think we’re ready for it. I won’t have sex without birth control, and I’m afraid to ask for it. I’m only fifteen, and I come from a Catholic family. However, I don’t know how my mom feels about sex, but I do know she’s okay with birth control. I’m really afraid she’s going to freak out. How should I approach her and ask?