if she's a virgin you need to start working on her now if you're ever to have sex...if she's never had anything in her it's going to hurt her more than likely, and also before you go to do it you need to get her really wet and horny and finger her for awhile first to loosen up the muscles some...take it slow, and remember it's her decision, if you go to have sex and it hurts her, make sure to hold her lots and shit because most girls feel like failures after they have to tell the guy to stop. make sure you tell her before anything happens that if it does hurt, that you will stop when she says to and you won't be upset or angry and you understand her position. 15-18 year old girls are some of the most emotional people i've ever met >.<