The most important thing to keep in mind when auditioning is that 
you are selling yourself.  Pick a monolog that you like, one that 
shows true emotion, and allows you to demonstrate your abilities as 
an actor.  Make sure that 
you are speaking to someone in your monolog (your mother, your best 
friend, whatever the monolog requires) and that you have a clear 
idea of your relationship with that person.  Obviously that person 
isn't really there, but imagine where they are standing and how they 
are reacting to what you are saying.
Auditions can be intimidating, and most auditioners are very 
nervous.  Don't worry about that, it's normal.  Use that energy. 

The most important thing is to learn something from every audition.  
Did you do well?  Why or why not?  How did the others in your group 
do?  What worked for them?  What didn't?  Can you use any 
techniques 
you saw to make your auditions stronger?  Many people audition a lot 
before they get the parts that they want, but every audition is 
valuable.  Learn from them.