Don't worry, an ex boyfriend of mine had this problem the first time we tried it as he was so nervous about it.
Sorry for the personal question, but was he maybe not quite hard enough to go in or were you to tight to let him in?
Sounds like you were both just trying a bit to hard, don't worry, just relax and let it happen without trying to force it. Just just going at it gently, him gently pushing it in bit by bit in a nice rocking movement so you both get used to it gently rather than him just trying to stick it all right in in one go.
Don't rush it it's not a competition, take your time. A couple of drinks may help (depending on how old you are) to relax you, don't overdo it though as that can make things worse and you want it to be a nice experience and not one you're too drunk to remember or enjoy.
Play with each other lots first to get each other ready (don't just dive straight in), make sure he's hard enough and you're wet enough, maybe use some lubricant if it makes it easier, you can get it from Boots/Superdrug round where the condoms are.
How old are you if you don't mind me asking? Just it can be difficult if you've had to sneak somewhere and are at risk of being caught etc... That can put uneccesary pressure on and make things more difficult to relax.
Hope this helps : ) x