The International Residential Code (IRC) lays out a series of requirements for pouring a concrete slab on grade. Its more than just forms and pouring concrete!
The slab has to be at least 3.5″ thick and possibly thicker for expansive or other difficult soils.
The area within the foundation walls has to be free of vegetation, top soil or other foreign material. The fill that goes in must be compacted and no more than 24″ deep for sand or gravel or 8″ deep for earth. In addition, you need a base course of 4″ of clean, graded sand, gravel, crushed stone or slag under 2″ when the slab is below grade unless the existing soil is exempted by the soils report.
There needs to be a vapor retarder at least 6 mil between the base course and the slab. Its not required for accessory structure, storage rooms under 70 sf, flatwork that isn’t enclosed and other approved site conditions.
Reinforcing has to be supported so that it is in the top 1/2 to 1/3 of the depth of the slab throughout the pour.
Finally, the IRC doesn’t show this in the code section, but you may need insulation under the slab, that is verified with the IECC.
For this post I’ve pulled from the 2009 IRC, check with a qualified architect or engineer for requirements in your jurisdiction.