Now some tips about "the talk" itself:
Take it easy. Don't try to cover everything all at once. "The talk" can and should be several conversations. You'll want to have a talk about puberty much earlier than one about dating expectations, for example. Girls today begin puberty around the age of 10, and boys around the age of 12, but most parents don't want their 5th graders dating so that discussion can wait!
Make time. Don't try to squeeze these conversations into a few minutes driving to soccer practice. Consider regularly setting aside time to do something with your teen that he or she enjoys, and then ask a few intentional questions. The consistency of time spent together will make your teen feel more comfortable. Make sure you ask open-ended questions, not questions that you are expecting a certain answer to. Assure your teen that your conversation is confidential, and affirm your teen's feelings as legitimate.
Listen, don't just talk. Ask your teen about his or her friends, what other kids at school are doing; what kind of pressure he/she is facing. And really listen when your teen tells you about these things. Try not to act shocked, and consider not offering advice unless they ask for it. This will help create a safe space in your relationship for your teen to be honest, and over time he/she will probably open up more personally.