Find a scene with two people. When you audition for a role you usually read with a casting director or associate. Don't confuse the scene or read by having more than two people talking back and forth. (Sometimes this is unavoidable; however, you should always strive to choose a scene with two people if possible.)
Look for a scene that exhibits who you are as an actor. You may long to play Hamlet. However if you are a 15 year old female, you'll never get paid professionally to play that role. (Except in a new interpretation.) Choose an audition side, which is about your same age range and seems to go to your natural personality. Once you have established more of a career you can "stretch" to intensive roles that aren't "you."
Choose an audition side that has an equal part for both people with a good cadence of back and forth dialogue. The audition scene does you no good if the casting director is speaking the whole time. So, find a scene that focuses on you for the audition.
Pick a scene with conflict in it. (If the scene is well written, then it will exhibit some sort of conflict.) It can be comedic or dramatic conflict, but there needs to be two opposing points of view. This will provide you the best opportunity to show your strength in acting by winning your side of the argument.