Get headshots or 8 x 10 photographs of your face with a reputable photographer. If you are going to an audition on short notice and can't afford a professional photographer, take a Polaroid or other recent snapshot. Bring a resume that lists your acting experience, any acting classes you have taken and any special skills you have. Reputable headshot photographers in Los Angeles include Ken Weingat, Alan Weissman and Sara Corwin.
Read "Backstage West," a trade newspaper unique to Los Angeles, which you can buy at your local newsstand. "Backstage West" will have a list of auditions going on in the area. If your audition is an open call, you can just show up with your photo and resume, but if not, you will probably have to send your headshot and resume into them by mail before they decide if they want you to audition or not.
Once you have an audition appointment, you will probably be asked to prepare a monologue. Stick with a monologue that is meant for a character your age and one that is not too well-known. Do not use cursing or swearing in your monologue unless you are auditioning for a dark, rebellious teen character.
Show up to your audition with a parent, guardian or another trusted adult. It is important not to go to an audition alone, especially if the audition wasn't recommended by an agent or manager. Reputable agents in Los Angeles that represent teen actresses include, but are not limited to, Abrams Artists Agency, Morgan Agency and Creative Artists Agency.
Don't sign anything unless your parent or guardian is present. It is important to read all documents before signing or agreeing to anything. Many agents will try to scam uneducated parents, so do not put 100% of your trust in them, especially early in the relationship. Reputable talent management companies in Los Angeles include Allied Artists Management, Arc Talent Management and Kids Unlimited Talent.