Do's:
1) Get hyped up for it. Yeah, you'll soon find that sex is only sex, but hey, it's your first time! The more excited you are about it, the more SHE will feel the moment is meaningful; which it is.
2) Be yourself. If you cum too fast, fine. If you can't cum, fine. If you're perfect in bed the very first time, well, you and I share a very rare trait!
In all seriousness, don't worry. She's not comparing you enough to other people to make her actually care how well you do. She's also in the moment, too, y'know.
3) Wear a condom. You know the deal; check the expiration date, make sure it's not ripped, if it pops put on a new one, hold it close to the shaft when you're pulling out. Maybe even ask her what condoms she prefers; she might pick something out that makes your sexual experience all the more better.
Don'ts:
1) Don't be nervous. Nervousness will just make you even more nervous, and then at the end of it all, you'll feel like you did something wrong. When in fact you did: You were nervous! Just stay calm, and as I said, be yourself.
2) Don't go raw. Your penis is precious. So is your life, as is hers. I'm sure you don't want to be hearing "Waaah waaah" in 9 months, and I'm even more sure you don't want to be another beneficiary of the AIDS walk. Just like everything else in life, think it through before you do. Hey, that rhymes!
3) Don't ask her how you're doing. Your confidence is one thing that will drive her yearning for you in and out of the bedroom. You know you're good, so she knows you're good.
One last bonus DO:
1) If the sex is great, let her know. It will be encouragement for her to let you have sex with her again and again and again.
Enjoy!