First up, you and your boyfriend should make absolutely sure you are ready for sex and all the responsibilities that come with it. Both of you should sit down together and read Ready or Not? The Scarleteen Sex Readiness Checklist and decide if you have what it take to keep up a safe, happy and responsible sex life.
If you are totally sure you pass all the criteria, then you take all the proper precautions when engaging in sexual activity. That means using condoms and water-based lube. Hormonal birth control, however, is a no-go because you haven't started your period yet. Even though you haven't started your period yet, you have to use condoms because there's always that chance that you might start ovulating around the time you have sex. Have your bases (and his penis) covered. For more information on how to protect yourself during sex, read Safe, Sound & Sexy: A Safer Sex How-To
And pay special attention to the possibility of infections. You are younger and less-developed, and this means that you are especially susceptible to infections (particularly inflammations of the cervix). Perfect and consistent condom use will be your best protection against infections.
One more thing, make sure you can see a doctor. Now that you will be sexually active, you will need to be up-front and honest with your doctor. Your doctor needs to know that you are sexually active so s/he will know to examine you for certain things on a regular basis. Your doctor will even recommend that you have a pelvic exam. While it may make you uneasy, it's all part of ensuring you stay healthy.