I can't usually have an orgasm without a little extra help. Find a position where you can stroke yourself while he's inside you. Doggy style usually works best. I didn't really start having a vaginal orgasm (with the g spot) until I was about 29. I think it just takes a while to kick in for some people. You might want to try poking around with your finger or a vibrator to find out where your g spot is. Once you find it, you should then also be able to direct him to it. The suggestion about getting eaten out before hand is a really good idea too. It'll help your boyfriend know his way around you and what you like. Also, it'll make you more "engorged" and once he penetrates you you'll feel it much more.
I just read
here that you were sexually abused as a child. This may almost definitely be at the root of the problem. I would suggest therapy. Does your boyfriend know? Perhaps that would relieve some of the subconscious psychological tenseness that you may be experiencing.