1. Dont forget to bring your girlfriend
2. foreplay! don't rush into penetration if you want it to be fun
3. be hard
4. if u dont know where the hole is, let her place it in for you while you keep a poker face (act as if you knew)
5. dont c*m too fast ... if you feel it coming, slow down and make the whole experience longer
6. throw in a joke if you feel the atmosphere is too tense: i dunno, when u go looking for condoms and need to get off her for a second, act as if you are confused about which way to go and where to place your hands, and place your hands repeatedly on her boobs and crotch by excusing yourself and seeming like you didnt mean to do that. make the confusion last for 30 seconds or so, and if done properly, she might find that amusing, while you get some good contact with her nicer body parts
Most importantly, try to enjoy and dont think about performing and all that stressfull stuff. Enjoy and report back!