the first time i was seconds away from saying 'oh my god! stop that really hurts' but i let it go and ended up really enjoying it (i think i am one of the few people on the planet who doesn't have a horific 'first time' story)
the thing to remind her is that its a different kind of pain. you know how that say theres a fine line between pleasure and pain? well sex can kind of blur that line
i'd also agree with what other people have said, make sure you go really slowly, that she is really lubricated, that shes relaxed and not all tensed up. also, be prepared to stop if she freaks out, you sound like that sort of guy who wouldn't want to do it if you didn't both want to
hope it all works out