You will have to start out with amateur events as a proving ground. Once you get to know the officials around your area they can help you get into the pros. Unless you already know people with your commission that will go out of their way, this is the way you should go about becoming a judge. This is from usaboxing.org
Q. How do I become a USA Boxing official?
A. To become an official, which includes referees, judges, timekeepers, tournament directors, weight masters, inspectors and draw masters, you need to join USA Boxing as an official member by registering with your LBC. You must also be certified by your LBC. These actions need to take place in order for you to serve at any USA Boxing sanctioned event.
Here is a link where you can see who to contact. http://www.usaboxing.org/90.htm
It also has a basic rules and hints for beginning officials link.