Contracts just the terms of your agreement, outlined in writing. What the sellers (you) responsibilities are, what the buyers (them) responsibilities are and a payment schedule.
There's probably books with example contracts at your local library and definately should be some at your local Barnes & Noble (or whatever bookstore). Even if you pay a lawyer to write one up for you, still go do a little research just so you have an idea of what they look like.
They can be simple and straight to the point. Bigger contracts with lots of clauses are really only necessary when you're dealing with major artists/labels.