I totally agree with that guy above, it will come natural to you. But here is some things you should AVOID when your big moment comes.
1.) Do not put pressure on the situation to "just get it over with". This is your moment, not his. He apparently has already had sex
2.) Don't put too much pressure on yourself to make it the "perfect moment". Your going to go through alot of emotions at once being exposed to that kind of environment; fear, excitement, anxiety and being overwhelmed are all possible. Try and relax as much as you can and if you start feeling overwhelmed just slow down a little bit.
3.) Keep to the basics! You don't need to WOW him the first time, just the fact that your underneath him is probably enough for him. No tricks, no crazy positions, but don't just lie there either. You can do alot with your legs and your hips with him on top to make things better or tease him. These are very basic moves and you don't need practice for um. Go for it!
4.) Since this is your first time, make use of #3 to control the amount of penetration you receive. If he goes to deep and it starts to hurt then don't be afraid to tilt your hips back a bit to give him a signal. This is what I like to call sex body language. If your tilted up, he can go deep, if your tilted down he can pull back. This also will come natural to you but still, it's better to figure it out now then the second or third time (which might immediately follow :) ).
5.) Don't forget to breathe! If you breathe too fast you might hyperventilate and if you forget you might pass out!
Happy Hunting!
p.s. I was raped when I was 12, had the mans baby and gave it up for adoption when he was 2 days old in the hospital. I had no emotional troubles with the first time and my advice; look into his eyes and tell him that your ready and if there is anything HE feels uncomfortable about with you being abused, that he can talk to YOU about them. Sometimes men are too careful. Thats when us women put them in line. If you have truly dealt with it and are OK that doesn't necessarily mean he will be too. Talk to him. Communication in the end SOLVES EVERYTHING!.