Do not show discomfort during your conversation.
Keep your mind open - your teenager might know things that you were not aware of. You might learn something new.
Talk about STDs, but do not be harsh. That might cause your teenager to close up and distance herself from the conversation.
Talk to your teenager about safe sex and abstinence. Also consider the possibility that your teenager might already be practicing sex.
Let your teenager know that she can talk to you about anything. Be there for her by considering all her issues.