When you do have sex, keep in mind that you do not have to be ovulating to get pregnant. You should always be using at least a condom when you have intercourse; the "withdrawal method" (having the guy pull his penis out before he comes) is only 3% effective in preventing pregnancy, making it the least protective method.

If you have completely unprotected sex again look into taking Plan B as soon as possible afterward. You can get it over the counter since you are 18. It's expensive (about $50 out of pocket), but less expensive than a baby or an abortion.