well, how old are you? i kinda had the same problem as early as a couple of years back, i'm 20 now. recently i've been able to pull it back past the 'head' but i ran into another problem where my frenulum (connects the foreskin to the head) is too short. i'm actually going to go see a urologist about this this week. procedure is supposed to be pretty minor. if what you're talking about however is phimosis, then it might be a little more complicated. from what you said (not being to pull it back past the head) it sounds like that's what it is. there are a couple of treatments from circumcision (ouch?) to creams and stretching and such. do a search on the web or try checking out this place:
http://www.male-initiation.net/conditions.html
it's got a lot of good info and some actual accounts. i'd suggest you go see a doctor about it too, it's probably what's best.