It will build up a lot of character in you if you do this yourself.
Therefore, get yourself some information on forming up concrete. The most important thing is to establish a good grade. I see a lot of garage slabs that are far too low. I like to see a 1 1/2" lip from the driveway to the garage slab.
I like 4" of compacted road stone under the slab. You can have a 1'x1' perimeter with one piece of 5/8" re-rod in it. Put some plastic down before you pour.
Now, in order to do this easily, I would run a form down the center. So you'll be pouring two 8x24 slabs. In this form down the middle, drill yourself a 5/8" hole every 2 feet, and cut yourself some 2' long pieces of 1/2" rebar. Insert these dowels, then remove the form when the concrete has set up. Place the other half, using the existing concrete as your form.
That should take care of your slab. You'll need a compactor, some concrete hoes which you can also use to move your rock, some form stakes and forms, line to lay it out, etc. You'll need a screed board to screed the concrete (a straight 2x6 or 2x8 will work), a bull float, some concrete finishing trowels and some pieces of 2" polystyrene to kneel on as you go through and trowel the surface.
If you can reach out 4' from the sides, you won't need to get out on the concrete to finish it. If you plan on doing more concrete work, you can get a trowel to go on your pole which you use for your bull float.
You can either score the concrete every 8' as you place it, or cut it later. If you want to segment it, then you'll need a segmenter and probably one to mount on a stick. You can cut it with a masonry blade in a circular saw. Just have a few blades on hand. Cut a 4" slab 1" deep.
Cure the slab properly. Try to place the concrete in cool weather.
Oh, I always use concrete with fiber added and I wouldn't put any re-rod in the slab, other than in the perimeter and the dowels.
