You have been given some great advice Just relax and make sure you are ready by using foreplay and lubricant also is great! Just relax and your body will adjust :)
Don't straight to sex right away let him have fun with you the more your wet an relaxed the more it won't hurt him relax enjoy everything the more you enjoy it the more he can without worrying if your going to be to tight
I am no expert but I do have some advice. Use lots of lubricant and go slowly, plenty of foreplay using fingers and toys and work your way up to sex slowly and gently.