Though there's nothing terribly complicated about forming an S-Corp or a LLC (the two entities you'd be likely to form), there are a lot of little niggles that, should you get wrong, can go very wrong. Allow me to remind you of this disaster that happened on HN just a few days ago: http://news.ycombinator.com/item?id=2399139
tl;dr Don't hack your legal stuff. Get a real lawyer.
My current startup, Munch On Me, was going to be much bigger and I needed several contracts drafted with founders and such. I knew I wanted funding eventually so I set it up as a C Corporation from the start. Legal fees are in the several thousands for a good lawyer. There was a lot of customization in the process, which is why it costs so much as opposed to a one stop shop like LegalZoom.