Don't cause yourself any unnecessary stress. Show up for 10-15 minutes early and be prepared to wait. Once you've made it to the audition, you want the casting director to remember you. If you've been given lines to read ahead of time, rehearse them with anyone you can.
Being unprepared won't score you leads in feature films! Try not to be nervous. We know you're bound to be anxious, but just remember that casting directors are people too and you'll do a much better job if you're relaxed.
Walk in with a big smile and a lot of confidence. Nothing blows casting directors away more than actors and models who exude self-confidence.
Practice makes perfect!
If you have the opportunity to take a drama/acting class at school or at your local community center, do it. Just like in sports, you have to keep stretching and training your acting muscle if you want to land that big part.