Dear (Another) Anonymous Girl,
I think it is really nice of you for caring about having your first time go just right. To be honest though, there is a good chance it won’t be just as you imagined.
With that being said, I think that you and your girlfriend just need to be open with each other and tell each other if something is not feeling good.
As far as you both achieving an orgasm, do you masturbate? If so, what gets you off then? If you know what you like tell her! And you could ask her if she likes anything in particular too. If you still have nothing to go off of, just experiment and have fun.
Everybody gets nervous, but just stay calm and communicate with your partner.
Hope all goes well!