From your post, I gather that this will be your, and possibly her first time with anyone. If this is the case, the experience can easily turn into something you'd rather forget, by following a few simple rules though, it can be a great memory.
You're nervous, that's normal, she probably is too. Trying to be cool will most likely make you look like a jackass. Being yourself is always the best policy, it may even make her feel better to know that you're nervous too. And if you're so nervous that you stumble all over yourself you may need to slow down, or even back up a bit.
It doesn't have to happen tonight.
Go slow and don't force things. If you can't both relax you won't have a good time. If she's not ready, be a gentleman, treat her like you always have even though inside you want to scream. Once you get home you can jerk off, you'll get another chance, and she'll appreciate your patience and understanding.
Make this about her.
Rest assured, you'll get yours, the same can't be said for your girlfriend. Always make sure she's ready for you first, and don't ever give up until your lover cums too...at least once.
Foreplay foreplay foreplay.
It really is important. Spend a lot of time on this, treat the actual sex as an afterthought. And never kiss her pussy harder than you'd kiss her mouth.
It would really suck if you got your girlfriend pregnant. Plus wearing a rubber will make you last longer.
Your buddies don't need to know shit!
Bragging to your friends might make you feel like a stud, but it'll make her look, sound, and probably feel cheap. Experience has taught me that your friends WILL hear about it...if they're friends with her friends. And if it gets back that you've already told your friends it will likely end badly. Yes it's a double standard. That's just the way it is. In addition, her friends will also most likely get a full account of your performance, so that's another incentive to be good to her. Luckily though, women can be very forgiving if you treat them right. So if you're trying to please her, but fail at it, you're still likely to get a good report.
Good luck jonathan