I've been a singer since before I could talk, so I'm well aware of the pre-audition jitters. I've also been in a prestigious chamber choir for quite a while, so I must be doing something correctly, right?
Anyway, this is something you can do right at your desk. Stand up and just take a breath like you are about to sing something. You don't have to sing at this point, but just take in that first breath. OK, now do it again and pay attention to your shoulders. Did they move at all? If the answer is yes, then you aren't breathing properly!
It seems so simple, but that first breath is very important for singing. You want to be breathing from your diaphragm, not from your chest. Your diaphragm is located right below your lungs, and can be used to draw a much more effective breath. It will help you sing louder, longer, and with more pitch control by giving you something called "breath support". This means that, by using your diaphragm, your breathing will be much more supported and efficient, coordinating your breaths in time to the notes that you're singing.
To breathe from your diaphragm, start by putting your hand on your stomach. Now, without using your stomach at all, breathe in so that your torso expands. Breathe in deeply, and make sure that your shoulders don't move. If you did it properly, you should see your hand move as you breathe in. Next, exhale. As you do this, your hand should sink back in to your stomach. Do this a couple of times, and practice singing solid notes after breathing in this way. Next, work your way up to singing your particular part.
This is the a very common mistake that beginning singers make, at least in my experience. breath control does take a while to get the hang of, but it should give you an edge at least for your audition. If you want some more help with specifics, notes, pitch and volume control, or in reading and playing your music on the piano, don't be afraid to shoot me a message. Hope this helps, and good luck!