Having had lots of experience in this arena, not only do I endorse others' suggestions of outsourcing the concrete, but have them take care of the footings, forms and rebar. Not only will you avoid a potential he said/she said conflict if anything goes wrong later, but these crews are fast, efficient and the savings of doing all the prework yourself is minimal.
Depending on your location, a slab your size could be done as a monolithic pour which would save a lot of dollars over having frost footings dug and poured. Draw up your plans, specify stem wall thickness and height, slab thickness, mesh or rebar (and spacing, vapor barrier, any in floor PEX and openings for your doors and get a few quotes. Don't forget to ID number and spacing of anchor bolts for your sill plates, and where you want the wall penetration for elec service. If the location is pretty level and won't need much fill or leveling, you could shop your plan around as-is and come in pretty close. Don't forget about any poured aprons- now's the time.
The trend these days is to put a very minor slope from rear of the garage to the front to help with drainage, especially in snow country. Also verify where they plan to make the cement cuts. Anymore, I put these on my drawings myself as the job may fall to someone who doesn't understand why they need to be in specific locations.
