We won't tell you what to say, because that is obviously very much up to you. You probably want to know how you can make sex a topic you are both comfortable discussing, and how you can present information about sex without it seeming you are actually encouraging your teen to lose her or his virginity.
Our first tip is one it will already be too late for if you have a teenaged child now, but we'll mention it anyway. It is to start talking about sex from an early age. At first, your young toddler may have questions about how babies are born and made, especially when they are about to get a new sibling. Later on, they may wonder about the mechanics more. If you are a Christian or other religious person, you may tell your child that sex is something married people can enjoy together.
If you are more secular, saying that sex is a very serious emotional commitment between two people, which is also pleasurable, is an option. Making your home environment one in which anything anything at all can be discussed openly, without also condoning it, is a positive step. You can buy basic biology books that feature reproduction and menstruation if you want a more technical approach, and talk about having sex in the wider context of puberty and growing up.
We also strongly suggest that you give your child information about sexually transmitted diseases how one can be infected, and what can be done to prevent it (from condoms to faithful sex inside marriage). Knowing what options there are to stay safe(r) doesn't necessarily encourage sex at all. Also know that sex will be a topic of discussion among peers. Back when I was a teen, the following bits of disinformation went around:You can't get pregnant if the man withdraws before ejaculation. Menstruation occurs when the egg is released (obviously false!) You can't get an STD if you only have oral sex.
If you want to make sure that your teen has a good pool of accurate information to rely on, it's got to come from you, not peers! I also personally strongly believe in teaching boys as much about the meaning of sex, its dangers (diseases as well as unplanned pregnancy), and the responsibility that having sex carries, as girls even though they are the ones who can get pregnant.