Checkout these online resources as eHow:
Acting Audition Preparations
For our auditions it is a good idea to come prepared with a short, memorized monologue. Your piece could be anywhere from 30 seconds to 2 minutes in length. Checkout this monologue advice article on Ask.com all about Performing a Monologue.
How do you find a monologue to perform? Check out these tips on How to Pick a Monologue that Works.
And - of course - BREAK A LEG!