Next time (if there is a next time with this girl), take more time over it and try it in the female superior position. You do your best to lie still and let her do any pushing. If you're both young healthy, and (importantly) both really into each other, there really shouldn't be a lube probelm, but as people have said use some lube if necessary. Once she's got you inside her, let her set the pace, at least until you can tell she's relaxed and properly enjoying herself. Good luck.