penis foreskin won't slide down

You"d better see a urologist because if it really tight and you try to pull it by yourself by force it may go under your penis head and press it causing you severe pain and not be able to return in which case you'd better go to ER. On the other hand you may try gradully pulling the foreskin each time more and more this way making it more elastic. But the best way is that a doctor sees you!