Practice. It sounds obvious, but you can never get too much practice. Practice the audition material every day until the audition. Fight off those last-minute jitters by practicing some more. Your confidence will show in the audition.
Research the show. View several versions of the musical your high school is planning to produce. Become familiar with the key musical numbers. Get to know the characters and their nuances that make the plot work.
Get into character. Start to think like the lead character in the musical. Adopt a walk, a prop or a way of talking that fits the role you want to play. Walk around school in that character as much as you can. It's good practice for you and other people will start to see you as the obvious choice to play the role.
Be a good sport. The teachers who make the final decisions about casting your high school musical are obviously looking for students who can play the parts, but they're also looking for students who work well together. Anyone who's been around musical rehearsals knows problems develop quickly when there are prima-donnas involved. Avoid being one of those prima-donnas. Keep your competitive spirit under wraps and support everyone's efforts to shine in their auditions.