How do people become qualified to work as caterers?

Make sure you have all of your licenses required by your city/state, know what you want to prepare and contact the health department about your new business (better to be prepared than surprised), purchase all of the necessary equipment, create a contract (I suggest you charge 3x the food costs), and you may want to consider culinary school if you aren't already an amazing cook.

Try these sites for more info:

- Entrepreneur
http://www.entrepreneur.com/marketing/marketingbasics/marketingplan/article43018.html

- Global Gourmet
http://www.globalgourmet.com/food/egg/egg1296/eggtrick.html

- Mission Restaurant Supply
http://www.missionrs.com/catering-supplies.html

- Business Licenses
http://www.businesslicenses.com/

Hope this helps!