First time having sex

Sarah
I'm 17 and my bf is 17 and we've been dating for 6 months now. The other night we tried to have sex for the first time, (after he put on a condom) he tried to put it in but no matter what it wouldn't go in; it kept sliding around. (He might not have been as hard as he could've been though)
I've never been able to get more than one finger "up there" when im by myself so idk how his penis is supposed to fit. Any tips for when we try again?