How do I tell my grandma I want to get on birth control?

I want to get on birth control . I am sexually active but my grandma doesn't know . But I also want to get on birth control to balance out my hormones so I'll stop having really bad acne breakouts. I don't want to tell her it's for my acne because I know she'll pull the whole "sex" card out on me and start questioning me . I'm 16 years old . So what can I legally do if she tells me I can't get on birth control ? I have a way to pay for it on my own . But I just don't know how to break it to her