Most Helpful Guy
Smile! Remind yourself that it's not a do or die situation, look at it as "I'm going to audition for 10 musicals in order to get accepted for 1", so this is just one of MANY auditions you will make. Of course, your goal is to get it. But understand that life is a numbers game... You try a certain number of times and it works out once. All it means is to try out for a whole bunch of musicals or plays or whatever it is you're doing (apply to a whole bunch of schools, etc.), and don't ever put all your eggs in one basket.
By taking this approach, hopefully you'll feel less pressure and be more relaxed, which will actually make you perform better and increase your chances of getting the part.
Have fun, do your best, take deep breaths, and don't worry if you don't get it! There will be many many more opportunities after this one!
That being said, good luck! Hope you nail it and get the part! :-)