Check the school play audition guidelines. Some auditions require you to prepare a piece to present, while others will set you up with a cold reading--they will hand you a script on the day of the audition and ask you to read it. You may also need to schedule an audition slot.
Choose an audition piece. Try to match your piece to the actual play. If the play is a comedy, prepare a comedic piece. Keep your piece simple, especially if you need to prepare a musical number.
Practice speaking in front of people. If you are preparing a piece, ask friends and family to listen to you act it out. If you will have a cold reading, practice reading unfamiliar material in front of your family or anyone you can recruit to listen.
Relax the evening before your audition. If you have been preparing up until this point, you should be able to sleep well and go to your audition calmly, ready to try out.
Run through your piece once just before you audition. Keep calm and focus on doing your best during the actual audition. If you do a cold reading with other actors, concentrate on what your character is doing more than what he is saying. Pick a verb and play that. For example, if you think a character is trying to PERSUADE someone to do something, play the scene with the intent of persuading the other character. Step back at times to see if your words are persuading the character. Focus on the doing of the scene rather than emoting, which leads to bad acting.
Contact the director after your audition to find out the results. Typically, a director will give you a set time to check back with him or her. If you don't get the role, take the opportunity to ask where you can improve, as well as if there are opportunities to participate in other ways.