You should try to choose a piece that shows off your skills at their current level.
Be careful not to select a piece that is too easy, or too challenging to learn in time for the audition.
You will need to provide copies of the written sheet music for your selection at your audition.
Don't just play notes over and over again - efficient practice starts with a plan.
Know how long you have to learn your piece, and make a plan to stay on target. Your should be to have your piece(s) audition ready at least one week before the actual audition.
As you get closer to your audition, play for others and seek feedback. You can play for your music teacher at school, your friends, or even record yourself and listen back!