Foreskin

To answer this in a useful way, more information is needed. Let's assume a few things and go from there. Assume you are a boy aged before or around puberty or early teens, and that your foreskin is "stuck" to the penis head so that it will not retract properly. This can be due to two different things, either latent childhood adhesions, or phimosis. In the latter case, it isn't really stuck, it's just that its too tight or that the opening at the tip is too small.

In both cases, gentle tugging and stretching on a daily basis can help loosen it. The foreskin will usually not retract on its own, but requires persuasion. Once retracted, it will not necessarily stay retracted ("sit under the head"). That may need persuasion too.

If you have not already been instructed on this by now, then you should take initiative and begin working on it. It is important to get this done at an early age, because an unretracted foreskin can often choke and retard proper penis head growth and development. It can cause other problems too and may then require circumcision.