Bring a resume and a head shot
Even if you have no roles to list, you still need a resume. Your resume should include classes and courses, interests and roles in productions (if you have any). You should list the directors that you have worked with, and your teachers. Your resume should have all of your current contact information on it. You might not get a role in this show, but you might be the right type for another show later on. Directors do keep resumes and head shots on file.
Your head shot should look like you
Put your name and contact info on the back, in case the photo gets seperated from the resume. If you are auditioning for a community theatre, you will not need to have professional headshots. In either case (amateur or professional productions) your head shot should actually look like you. It can be in black and white or in colour, but your head shot should be very natural-looking. Glamour shots, or mood lighting, or artistic photos are not head shots. Avoid excessive make-up. A head shot is used to remind the director of your type. Directors sometimes get hundreds of auditioners, a good picture of your real look is important.
Give them what they ask for.
If the audition notice asks for a two minute comedic Shakesperian monologue, then that is what you should prepare. It is not helpful to casting directors if you are delivering Shakespeare and they are casting a Neil Simon play. It is not helpful to give a tragic monologue when the audition notice requested comedy. There are directors out there who will discount your audition entirely (now and in future) if you fail to provide what is specifically asked for in the audition notice. If you don't follow the directions on an audition notice, then it can be assumed that you will not follow direction at all.
Do your homework.
Understand your monologue and its relevance to a role and the play it is from. Read the play and understand it.
Directors will often ask about the piece that they are about to see, and you must understand that sometimes, the director is familiar with the play that your monologue is from, and sometimes they are not. They are asking to see if YOU understand what your character is saying and why. If your only reply to 'What do you have for us?' is, "This is a monologue from "The Odd Couple" , then you are in trouble. You should be able to say, 'This is Oscar's monologue from "The Odd Couple" by Neil Simon. He has just been kicked out of his house by his wife, and he is talking to his new room mate, Felix Unger, who is a very tidy man."
Read the play that you are auditioning for and understand its style. Try to deliver a monologue that is similar in style to that play. Is is poetic? Modern? Classical? Shakespeare? Wilde?
Your personal presentation
Don't chew gum. Yuck.
Don't bring drinks other than water. No half-caff lattes, no Coke, no Pepsi.
Don't dress inappropriately. If your clothes upstage you, you're in trouble.
Keep your hair off of your face. We want to see you.
Answer questions enthusiastically, tell short anecdotes, but don't go overboard talking about yourself.
Don't use profanity.
Don't come to an audition under the influence of drugs or alcohol.
Wear something that you can move in. You may be asked to dance or climb or jump or lay on the floor. Don't let your choice of clothes let you down.
Be as charming as you can naturally be. Everyone wants to work with warm, positive people.
Never give up
Most actors don't succeed because they stop auditioning. It may take 100 auditions before you land a tiny role as 'girl with pencil'. It may take more. The only guarantee in this business is that if you stop auditioning, you will not get roles. Directors love to work with talented, motivated actors, but they actually end up working with actors who show up, on time, and who audition all of the time. Talent is meaningless if it never leaves the house.
Remember that if you are not chosen for a role this time, it doesn't mean that you are not liked. Casting a show has to look at the entire cast a unit, and all of the pieces have to fit together.