If you are looking to save a few dollars by purchasing used garden pots or if you want to recycle those you have, you should learn how to sterilize them before you use them. You don't want to take the chance that the last plant in the pot had a disease that will transfer to your new plant. A little bit of bleach and some elbow grease is all you need to sterilize old pots.
Fill your bathtub up with 10 to 20 gallons of hot water. Use 10 gallons if you only have a few pots to clean and 20 if you have more pots to clean.
Add 1 gallon of bleach to water for every 10 gallons of water that you use--1 gallon of bleach for 10 gallons of water, 2 gallons of bleach for 20 gallons of water.
Set your pots in the bleach water, and let them soak for a few minutes.
Scrub the dirt off of the pots with a soft-bristle brush. After the pots have soaked, this should be quite easy.
Drain the bathtub, and rinse each pot with warm running water. Set the pots outside in the sun to dry.