birth control 18

I'm know my mom knows I've had sex with my previous 2 boyfriends, however she does not know I'm having sex with this other guy I've been sneaking in and out and we used a plan b just the other day because of 5x unprotected sex in 2 days, I'm 18 and I found a place that will do free depo. How do I tell her I'm going to put myself on it without telling her I'm sexually active and we aren't using protection ?