What you've got is called phismosis. If it doesn't cause discomfort when you're erect, or otherwise, it's probably nothing to worry about. There are a few options if you should decide you want to do something about it(or if it becomes painful):

1. Stretching, carefully
2. Your physician can surgically loosen the foreskin

or 3: circumcision.

Contrary to popular belief(thank you John Kellog) the foreskin serves a very useful purpose-- I know I wouldn't want to lose mine.