The first place to start would be to buy USAG's book with the compulsory routines (available on the USAG website). After that, it's a matter of learning the routines, the general deductions, and the deductions on each skill. The compulsory test only covers L5/6, but you need to be comfortable with L4 and possibly L2/3 (depending on your state). It would be helpful to know the shorthand, but it's not necessary to pass the test. Once you have the deductions and routines under control, you should be okay to test. I'd also highly recommend practice judging (especially with a current judge, even if you have to go to another club-- most are pretty welcoming to new judges) before you're assigned to a meet.