Do you actually have feelings for the girl? (as in, are you "in love" or do you think you love her?) If so, express them at some point before you get naked, but it might seem lame if you do so right before you penetrate her.
If you don't, well, just be nice, try to make the experience mutually satisfying, but don't fret over how often to change positions or whatever. Maybe just ask her what she wants and is interested in? Don't treat the whole experience like she is a tree with a conveniently placed hole and you are just plugging away to get your rocks off.
It's probably a good idea to talk about birth control / disease prevention stuff before you get going. There are enough unwanted **** in the world as is, and nobody likes getting a surprise STD...
Be respectful. The rest should go fine. It will probably feel really good, but if your experience matches that of myself and a lot of guys I know, the BEST sex is like the 3rd, 4th, 5th time, esp. if the girl is losing her virginity as well. After you both get the hang of it, it really becomes FUN, plus sexual ecstasy feels oh so good!
Good luck - don't be a jerk!