Whether you are auditioning for your next school recital, a talent competition, a theatre production, a television show, college, a choir, etc., you have the best chance of being considered if you walk in knowing exactly what you are going to do, and how you are going to do it. And this is going to take ample preparation and practice. That way you are going to feel confident walking into your audition, and it is going to show!
So let us take a good look at what it really means to be "prepared".
Step one is to find out if the audition you are going for matches your current skill level. Maybe you are the most popular singer/actor in your high school, but if you are going in for a TV singing competition, for example, there is a lot more to it than being able to carry a song decently. Do you know how to work a big stage? How to engage your audience? How to truly perform (vs. just "standing there")? Which songs showcase you at your best? Or how you are going to introduce yourself? Not to mention the meaning of the contracts many major auditions make you sign, or theatre plays, which additionally require acting and/or dancing skills.
The best way to do this is to consult with your voice teacher. If you do not have one yet, then that is the very first thing to arrange, and with ample time. A voice coach's worst nightmare is the person who calls to schedule "one lesson before my big audition next week" and "has never had voice lessons". There is not really that much we can do for you in just one lesson. It will help a little while you are taking the class, but the chances of you not only remembering, but memorizing and executing every tip with just a week's notice are not very high.
I have also heard of many high school musical theatre "stars" who think that they don't need help to prepare for college auditions, since they always get the lead part at school, and end up without a single acceptance letter. In many cases it is not for a lack of talent, but for a lack of thorough preparation and knowledge of the college auditioning process.
If you already have a voice teacher, then talk to him/her about your desire to audition, which project it is for, and how much time you have to prepare (ideally, months). Your teacher will help you determine if you are really ready for that type of audition, and help you prepare with repertoire, performance, introduction, etc. Personally, I like to aim for realistic auditions. Because the most confidence comes from showing off what you can do, rather than attempting to do what you are still working on. You work yourself up to more important auditions as you learn and grow.
Step two is to practice the entire audition from top to bottom, over and over, until it feels comfortable and natural. And by entire I mean from the moment they call your name until you walk out of the room at the end. Because the moment they call your name, all eyes will be on you, and first impressions are everything.
Let us consider a musical theatre audition, for example. Walk in with your head held high, a clean and appropriate outfit for the audition (you would never wear the same outfit to audition for a TV show than you should for theatre, for example), a big smile, and a confident air. Depending on the type of audition, you may have to hand over music track(s) or sheet music for an accompanist before you introduce yourself, and in the case of an accompanist, you should know how to explain exactly how and where you are going to start and end, for example. Have any required forms plus resume and professional head shot ready to hand over to the audition panel, if requested, and work out how to introduce yourself, your song(s), and your monologue(s) ahead of time. When you are ready to sing, signal the accompanist or the person handling the music track(s) that you are ready to begin. After you finish your song, thank the audition panel, collect your music from the accompanist, thank him/her, and leave confidently and swiftly. Your dance audition is usually separate, and you should prepare with your dance teacher for this. The monologue(s) should be prepared with your acting coach. The more you prepare each step of the auditioning process with your teachers, the better.
Nerves play a huge part in the auditioning process, and they are normal. But you will be much more nervous than you would be if you were well prepared, because you will be obsessing over everything that could go wrong. "Will I hit the high note?" or "I'm not sure what to say if the ask me about ..." are not the kinds of questions you want to have going through your head before your name is called, as they will make you feel insecure. So the more of those insecurities you can address through ample preparation and rehearsal, the better chances you will have of doing well at your audition.
Break a leg!