From what relatively little I know about Orks, I'm not sure you can get a 500 point list without going completely unbound for it, and even then it'd be a pretty weird list if you could. My suggestion is look at what you'll need for the Ork Horde formation (or whatever it is that lets you call Waaagh and all that), and then slowly start building that.
Also, if you're getting lots of Ork Boiz, do not underestimate the value of trucks. Yes, trucks can be shot to crap, but you're playing Orks - a horde army - which means you should be fielding lots of them.
My 2 cents. Hope that gives you an idea, but maybe someone who knows more about Orks can give a better suggestion