You have what is called paraphimosis, and it is a medical emergency. You should immediately go to the nearest emergency department of the nearest hospital. The tightness of the foreskin can cut off blood supply to the head of the penis, which can cause tissue damage and even cell death (necrosis) in the tissues of the penis head. You should seek emergency medical treatment to have this fixed and avoid medical complications.
Before you rush of to a doctor try just grabbing the head of your penis and squeezing it for a little while. that will force some of the blood out of it and make it smaller. that in turn will get it back under the foreskin. That said you would be wise to do some foreskin stretching exercises to enlarge the front of the fore skin. just look up "foreskin stretching " for a wealth of information.