While I don't yet have children, I'd like to think that I'd educate my teens about sex and the consequences of having sex at their age. The safest way to be in the know of what your teen is doing is to be open and honest with them. That includes going over safe sex practices, including how they can protect themselves from STDs and pregnancy. Since your daughter has already had sex, I think it'd be a great time to take her to see a gynecologist. Encourage her to ask the doctor questions and get a prescription for birth control if she thinks she will continue to have relations.