Select and memorize a classical or modern monologue. Avoid using monologues that are not from a play.
Choose a character within your age range. Most directors look for an actor who's physical appearance relates to the role.
Plan your audition wardrobe. Dressing in full costume is unnecessary, but do make an effort to look the part of your character.
Research the play you're auditioning for. Read the entire play and familiarize yourself with the setting, theme and characters.