An audition is essentially a job interview for an acting job. There are three main types of auditions: casting calls, casting auditions and theater auditions.
General auditions, referred to as casting calls, are open to anyone - experienced or not. You do not need an agent to audition at these calls, and ads for them are generally found in newspapers, magazines and Websites.
Because there are usually so many people attending, the casting director can only spend a limited time with each actor. Be sure to make a good first impression quickly. Also, you may be in line for several hours, so wear comfortable clothes and shoes.
Casting calls are most often used to help fill relatively minor roles. While these general auditions may not lead to an immediate role, they are a good way to meet casting directors for future opportunities.
Sides: Pages of the script that you will be reading from for an audition
Casting auditions or interviews are generally set up through an agent and tend to be for a more major role. Other actors at the audition may look very similar to you. The casting director usually knows what "look" they are after, so don't be put off.
If you do not have an agent, there is still a chance you can make it to a casting audition. A casting director may invite you after meeting you at a casting call. If you do not have an agent and somehow hear about a casting audition, send your resume and head shot to the casting director.
While the number of actors auditioning may be less than a casting call, the competition is usually fierce at these types of auditions.
Theater auditions are often advertised in local newspapers and magazines (and their respective Websites). Stay in touch with other local actors as well. Word of mouth is an excellent way to learn of auditions.
If you are auditioning for a role in a well-known play, take advantage and get a copy of the script beforehand to rehearse. Get familiar with the setting, time period, characters and other aspects of the piece.
Don't look directly at the casting director, or whoever may be evaluating you, while performing your monologue.
Many theater auditions will require a monologue. In selecting monologues, choose and memorize two different selections that will show your range as an actor. Ideally you should memorize a few different monologues so you can perform the most appropriate one at each audition.
Also, should the role you are trying out for involve singing, be prepared to sing one or two songs.
Be sure to stand in the light on the stage. If the casting director can't see you, he or she will have a hard time evaluating your performance and remembering you.