Bricks, Masonry, Asphalt and Concrete - Concrete pad/slab for basketball court

Sorry if this has been asked before, but I have searched most of the threads and cannot find the complete answer I am looking for. Here is my dilemma:

I am wanting to put a 19ft x 21ft concrete pad in our backyard that will primarily be used as a basketball court, and other kid-related games. No cars will ever be placed on this pad. I've been told by 2 other "do-it-yourselfers" that I can save a ton of money if I do any digging and forming myself and just have a contractor pour the concrete and smooth it out.

Here's my problem: What exactly do I need to do? One person tells me that I can just form it up and pour the concrete directly over the soil as long as I get the forms level and backfill around the form to close any gaps and prevent concrete from seaping under the forms. The other person tells me that I need to dig down enough for either 2inches of sand or 4inches of gravel. Which is correct? We live in Ohio.

Can you either give me the steps needed for my part in this project so that I can just call a contractor to pour the concrete, or is there a link somewhere that gives me detailed instructions for what I need? If you need anything else from me, just let me know.

Thanks very much in advance for anything you can give me!!!