If you act naturally, and treat people well, be the gentleman things should be ok. Remember first impressions count, so dress neatly.
Remember, if the adults are around they'll be expecting respect and manners, I'm a bit of a stickler for manners myself.
There's no reason to be nervous, as you already know her online so there's always that to fall back on for discussions. Have you thought about where you're all going to go? Movies? Coffee shop? Skating?
Find out what everyone's interested in and make that the starting topic for discussion. Music, movies, food, hobbies etc.
Good luck with it all.
