Just relax and realize everyone pretty much has the same junk with our clothes off. Stand in front of a mirror nude before you hop in the shower and just stare at yourself. Stare at your own body. That will help take the pressure off. Hopefully you’re not too old to think you have an aging, ill body. Then again, if you were old, you probably wouldn’t care about being nude, anyway.
If you were doing nude modeling for an art class, I can tell you most of them won’t care. (A few members of the opposite sex might laugh at you, but besides that, most won’t care and you won’t be their first nude model.)
I am FAR from confident with my body, but I wouldn’t care less about nude modeling. (I’d just feel sorry for the poor photographer who has to look at that!) I’m also assuming you’re a guy, so that’s even less to be worried about. Cup your balls if you feel you need to. If you were a woman though, it still wouldn’t matter much. Its probably easier being a female and modeling nude than male. More people would want to see you.
But either way, its nothing to be afraid of. Just don’t look down if it makes you nervous and speak to the photographer first. He/she will probably be used to nudity. After about five minutes, so will you.