How have you and your mom decided that you'll go on birth control? Unless you've specified it's to help with your period, your mother probably has a pretty good idea that you are already have sex - or you plan to in the near future.
It's up to you how comfortable you feel telling your mother that you're having sex. If she feels left out of your life and wants more details about what's going on with you, you could always start by telling her about school, activities, friends, etc. Sex is a big issue to start with, especially if the two of you aren't very big into sharing things with each other.
On the other hand, your mother has helped you to decide to go onto birth control. This might be a good opportunity to discuss sex - if you want to.
Parents can have a hard time realizing their child is growing up, but every parent has to ultimately accept the fact that their baby is dating/having sex/getting married/etc at some point. When that point happens is, for now, up to you.
And remember - the pill doesn't protect against STDs. You'll also need to use condoms for the first month you're on the pill, as it's not fully effective before than time frame.
Best of luck.