When do I talk to my mom about sex

Hi, this is my first time posting and well my boyfriend and I have been together for almost two years. We've been having sex for about 9 months now, with a condom of course. He talked to his parents about it last year while he was still in high school (he's a freshman in college now[we're two years apart]). But I've never really told my mom about it because I guess I'm afraid. My doctor knows and all but I was just wondering when the right time to tell her was. I was planning on telling her next year when I'm a senior  and asking her to be put on birth control since it was around the same time my sister asked her just to be on the safe side.