If you believe that it is okay for your son or daughter to be having sex, then you can pretty much leave them to it. If you believe that abstinence is best, then that's the message you should put across.
Either way, as soon as your daughter or son is sexually active, the message should be one of safety. You should talk about contraception and say that while the pill prevents pregnancy, it does not prevent sexually transmitted infections. Condom use is essential.
Do not assume your teenager knows about contraception. Read up on it yourself and share the information.
For your teenager to be open about their sex life, it is best to discuss it openly, even if it makes you uncomfortable.