Concrete footings provide the base for the whole porch structure. Footings should be at least twice the width of the posts that rest on them (usually at least 12" in diameter).
During the process of getting a permit, you should find out the required depth for footings. Wherever the ground freezes, footings need to be dug below the frost line to prevent frost upheaval. The depth may vary from 12" in southern climates, to 48" (or more) in northern climates.
Dig the footing holes using post hole diggers if footings are fairly deep. The diggers keep the holes uniform -- keeping concrete use to a minimum. If you have a lot of deep footings, you may want to rent a power auger.
TIP: Dig the hole wider near the bottom. This "bell" will help anchor the concrete in the ground.
Make sure to get a footing inspection, if required in your area. If you're hand-mixing concrete, make sure you know how much will be needed. We've learned that one 12x42 hole takes 10 bags to fill.
To prevent air pockets and to promote settling, fill each footing hole with about 1' of concrete, poke in with a shovel handle or other pole to help it settle and work out any air bubbles, then repeat until the hole is filled.
Fill each hole with concrete until it's level with the ground. Set a small 2x4 on edge, set a level on the 2x4 and screed the footing smooth. Then finish it with a hand float.
Let the footings set up and cure for a few days.
Consider putting down black plastic to prevent weed growth under the porch. You might also pour and level landscaping rock on top of the plastic to hold it down and hide it.