[seymojo536]

I also say hire it out. Concrete is the one project that if you do try to do it on your own and screw it up, you're stuck with it for a LONG time.
Find someone who does flat work for a living and buy him a couple of nice cold ones. He'll be glad to fill you in on just how much work you are looking at.

However, if you are dead set on trying to do this yourself, I would suggest you do it in several pours. Say do 10x16 or a quarter of the floor at a time. Start at the back and make your mistakes there. Find out what the mininum amount is the concrete company will send (some around here require 3 yards or they won't come) and how much extra this will cost you in delivery charges.

DO NOT pour on top of the topsoil. That needs to be scraped off along with 8-12 inches of the clay and a good base of 467 gravel laid down and compacted. Do plan on laying rebar and/or wire down and a vapor barrier under that. Put a couple of drains in now. It makes cleaning a whole lot easier.
And if you can find the money, lay down the pex pipe that you can hook up to a hot water heater and make the place cozy when the cold weather comes. Once that concrete dries it's a real bear to cut lines to put in a drain or water.