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A simple, "So I heard about you and Harley(?) at the party the other night. Who is this boy?" Would do perfect. But make sure that when you confront her about it that you make it clear that you aren't trying to lecture her about it but that you want to be there for her to talk to in case she needs to. Then just throw into the conversation that when it comes the time that there are certain things she needs to know about. Then tell her whatever you need to. She might not open to you right away about it, but she will end up being happy that you cared and was interested in knowing what was going on. Good luck.

Just bring up the party by asking her how it was, if she met anyone interesting etc. As for letting her know you're there for her just talk to her, let her know that you were once a teenage girl as well, and that you understand that she will be interested in kissing and such. I was about 13 or 14 and going on my first date, when my mom gave me a sex talk and is basically was: i'm here for you, ask me anything you want, and when you do decide to have sex use a condom and you'll get put on birth control.

Anyway if you're already close to her it shouldn't be too hard to approach the subject, just make sure you're open and honest with her, maybe share some of your not so great relationships, and some of the good ones. Seeing as she is going out with the boy that should make it a bit easier to talk to her about it, as apparently she does want some form of a relationship with him.