My first college audition went kinda awful. I was terrified the entire time, and don’t actually remember most of it. I wouldn’t wish those kind of nerves on anyone. Honestly, the thing that actually PREVENTS audition nerves in the first place is being prepared. Knowing what you’re doing is the best way to feel confident.
(If you don’t know what you’re doing, faking it helps.)
Rehearse in front of people. The worst stage fright usually happens because you’ve never performed a piece in front of someone before. You can kill that fear by actually, you know, performing it in front of someone. It can just be normal rehearsing, or you can sit your best friend or your parents in front of you and really run the piece. Both will help.
Get performance experience if you can. Even better than rehearsing is actually performing for a crowd, of course. Work with your band director or voice teacher or whomever and see if you can perform your pieces at a concert. If you can’t, just angle for a solo. ANY experience in front of an audience will help you feel more prepared for that audition.
Hydrate yourself. This might be a singer and woodwinds thing, but golly, being well-hydrated helps a lot. Have a water bottle with you. It gives you something to fidget with, too.
Drill scales and arpeggios (wind/brass/strings especially). Scales and arpeggios are large chunks of instrumental auditions, since they display technical skills. The stuff you should be proficient with should be listed among the requirements for the audition, so go and look at those, and ACTUALLY DRILL THEM. Don’t just run them twice and call it good. Add them to your regular practice. It’s good for you, like eating Brussels Sprouts.
Study your performance pieces’ histories. Singers especially are asked to talk about the larger work a piece is from, or to explain the story behind a poem. Know your translations, know your composers, and have at least a convincing fake theory behind why the composer did what she did with the work.
Do theory analysis of your pieces. This is more for your sake than for the judges’. However, knowing what’s a cadence and what isn’t can be helpful.
Remember that everyone who plays music professionally had a first audition too. They practiced just as hard, they panicked probably just as much, and they knew just as much as you do right now. You’re going to be auditioning for people who probably had an audition just like yours years ago. They auditioned for people who ALSO had a first audition. And so on and so forth, back until when auditions first became a thing. Your favorite performer had a first audition, and they survived. So will you.
Auditions can feel like the end of the world. They aren’t. You’ve got this.