Even though it's probably not an STD, take it to an STD clinic, which is usually free. Beware of doctors who think circumcision is the only answer for anything to do with the foreskin, though. Once you know what it is, you'll have a much better idea of what, if anything, needs to be done about it.
Skin issues can be many things often not a STD, but only a Dr can tell you what it is.