How do I prepare a hole for concrete?
Backfill the hole with dirt/fill to about 10-12" below the surface.  You should compact the soil frequently as you go - don't wait until the end.  Wetting it will help compact it.  After the soil add 6-8" of 1/2" crushed gravel.  Compact this.  Last, add 4-6" of concrete.

Unless you plan on parking something exceptionally heavy, you can use any concrete really - 2000-5000psi.