what should I do?

i want to start taking birth control and I know I'd have to go to my doctor and ask for some. but I don't want my mom to realize that I'd be sexually active now. I'm 16 and my boyfriend is 17. she rarely let's me see him because of the fear of “me having a kid now”. I don't want to have kids right now. not yet. I'm not sure how things will go when I ask her about BC. what should I do or say? thinking about this makes me have a lot of anxiety.....help!