In normal garage construction, the foundation wall around and under the garage is what keeps the soil under the garage from freezing, as will the soil in your area, to a depth of about 48" or more. Hence, the depth of the foundation is usually 48" or more. Without that wall, the soil under the garage may freeze, and thaw, and lift, and settle. So, your slab and the base under it will have to be designed to accomodate for that. Assuming that your garage is a one-car standard garage size, you'll want to excavate deep enough to place 8" of free draining gravel or 3/4" crushed stone. It will need to compacted very well with a a vibratory plate compactor. This base will need to be set at an elevation that will allow for a minimum 6" concrete slab reinforced with welded wire fabric (minimum WF6x6), and the fabric will need to be set at the mid depth of the slab. That's a minimum. Better still would be steel reinforcing bars in the slab...#4 bars set at 12" on center, each way, creating a grid inside the concrete. That is best left to a pro who is set up to provide, install, and tie the grid together, and then pour the slab. Some designers will also recommend what is called a turned-down-edge all around the slab. This is basically a thickening of the slab at the outside 12" or so, all around with extra reinforcing. The thickness here will reach upwards of twice the slab thickness. It's a good idea, but you can probably live without it. If the garage is a two car or larger, then consult a pro who can also provide the correct amount and location of jointing in the slab to control where the inevitable cracks will occur.
This kind of job isn't an easy one, but if you have someone with experience around, that will help...or hire someone.
[This message has been edited by treebeard (edited April 28, 2003).]