My foreskin is stuck 3/4 of the way down the HEAD of my penis and because of this I can't masturbate. What do I do
It's normal for you to not to be able to pull your foreskin back all the way until around 16 or so.
It should EASILY slide up and down the shaft as an adult.
Start stretching it. Get an erection and pull down on the shaft skin so the glans (head) spreads the opening OR use your fingers. It may be easier to do in the shower, the warm water helps soften the skin. You will find the glans very sensitive at first. You'll get used to it.
The more you do it the sooner you'll be able to pull it back all the way.