Though your main concerns regarding your teen and sex likely are sexually transmitted diseases (STDs) and pregnancy, focusing exclusively on these two topics won't get you terribly far when it comes to talking to your teen about sex.
If you start with these topics, your teen may just mentally retreat to some imaginary desert island that features promiscuous, repercussion-free, unsafe sex. While you certainly want to cover these topics, approach them in a manner that empowers your teen to make smart decisions. Attempting to use STDs and pregnancies as arguments against sexual behavior will likely fall on deaf ears, and these topics are too important to be ignored.
Don't make the risks of STDs and unwanted pregnancies the sole focus of the conversation.
Instead, present these topics well into your discussion about sex, as natural conversational topics that stem out of the subject as a whole. Let them know that ultimately they will be responsible for their own decisions about sex, but also that there's nothing terribly sexy about visiting a health clinic or dropping out of school to raise a baby.
Hold off on drilling your teen for information until you read the next section.