I'd tell her that her teenage years might be rough. I'd tell her not to worry about them and that life gets better in college, where you're surrounded by people that WANT to be there.
I'd tell her that she's going to feel a lot of pressure from the people around her to be "normal", and that's hogwash. She'll be told to look a certain way, act a certain way, and that there's a LOT of pressure to not be happy so she'll buy makeup or clothes so she can BE happy, but that that is BS and doesn't work.
I'd tell her that most of the people in the world don't know what they're doing with their lives. They're going through the motions and hoping no one else notices that they're lost. The people that make choices about what they want to do with their lives are the ones that are really happy, because their life has meaning. Tell her to do whatever she wants with her life, and you'll be happy if she's happy and lives her life well.
Tell her dating is a skill, and like any skill you learn it by fucking up a few times... Her first love won't be her last. There will be awkwardness and some small amount of stupidity, but you learn from that and you get better at it and eventually you figure out who you are, who you want to be, and then you find someone that you want to share your life with.
Tell her that another person can't complete her, ever. That's her job. She can't complete someone else. That's their job.
Other than that, I'd just spill your guts on random life lessons you've learned. :D Every word will be a treasure to her, so don't skimp on the trivial stuff!