Garage slabs rarely just sit on the ground. They are typically supported by a footer and stem wall system. The footer acts like an "anchor" for the building. It is a low, broad and thick concrete form that is poured below the frost line to prevent the building from moving with frost heaves and helps to support the weight of the finished structure. A stem wall may rise from the center of the footing to the level of the slab is the footing is too far beneath the slab to support it. You can dig a footer for a garage typically in a day.
Use a 100 foot tape measure to locate and place your stakes marking where the outside corners of your garage will be.
Run a string line from stake to stake so they are all connected. Keep the string within 6 inches of the ground. When you are done, you should be able to see the outline of the footing for your garage.
Spray the ground beneath the string with orange spray paint. Orange is recommended as it is the easiest color to see on soil or grass and in all kinds of light. Measure out from the string the width of the footing and spray another line. You should end up with two orange lines on the ground, the width of your footing.
Dig out the footing. You can do this by hand with a pick axe and shovel, but if you can use a machine like an excavator the process will go much faster. You must dig out the earth between your two orange lines and down deep enough to pass the frost line for your area. If you do not know what the typical depth of your frostline is, call your local weather station (radio or TV) and they will be able to tell you.
Use a shovel to trim the sides of your footing so they are straight and to level the bottom of your footing. Clean out as much loose dirt as you can.
Tamp the bottom of the footing for your garage using either a hand tamp or machine tamper. Machine tampers are sometimes called "compactors." When the bottom of your footing is level and compact, you are ready to order your concrete.