I'm guessing it'll be a skin contest :) In which case, before making one, I'd recommend you try and look at how other people did their contest. That way you'll get a good feel for how to set it up, judge info, price info, and what rules to put in :)

I'd also recommend you give this a read: 10 Steps to organizing a successful contest on PMC by DinowCookie.

And hey, if you have any questions, then I'm always here to answer :D
Well... If a builder can answer a skin-creators questions, but I'll do my best :p