It's NORMAL for you not to be able to pull back your foreskin until about 16 or so.
You can/should stretch it. Use your fingers to spread the opening OR get an erection and pull down on the shaft skin a bit, just enough to slightly stretch the foreskin. Don't force the foreskin back!
The more you stretch it, the sooner you'll be able to pull it back.
It may help to do it in the shower, the warm water can help loosen the skin.
Take your time, you don't have to hurt yourself. Again, don't force it back.