Summarize:

You can’t audition for American Idol without at least being a little familiar with the show. Look at old auditions online and check out what people sang. Pay especially close attention to what the successful singers sang. If they got through to the next round, they probably made a good song choice.  Try singing "At Last" by Etta James, like Kelly Clarkson did in her American Idol audition. If you're a fan of Katharine McPhee, sing "God Bless the Child" by Billie Holiday, like she did in her first audition. In order to pick a song for your audition, you first have to be aware of all the songs there are to choose from. Go for a long walk with your iPod and try to listen to as many of your favorite songs as you can. Keep the radio on in the car when you’re on the road. Cram in as much music as possible and chances are that you’ll hear something that inspires you.  "Kiss From a Rose" by Seal is a great choice for an audition song. It's often sung in the group round, so it's not frequently used for initial auditions. Be like past winner Jordin Sparks and sing "Because You Loved Me" by Celine Dion. All singers have a different range of notes they are able to hit comfortably – and some they are able to hit not so comfortably. For your audition song, pick something that sits comfortably within your vocal range. You’ll know it fits comfortably because you won’t have to strain to hit any of the notes and your voice will sound magical.  To see if you sound good singing the song, try singing it out loud, over and over. Practice it at home, and then when you’re ready, bust it out at karaoke. If you find a song you like, but it’s not in your range, take it to a music teacher and ask them to help you adjust the key. Think about the character you want to be cast as on the show. Do you want to be the underdog? The diva? The ingénue? The character you pick should be close to who you are in real life. When you know how you want to be seen, pick a song that corresponds with your character. If you want to be seen as the underdog, choose an uplifting power ballad. If you want to be the diva, pick something by Whitney Houston or Alicia Keys.  For a great Whitney Houston song, try “I Have Nothing.” For great Alicia Keys songs, try “No One” or “Fallin” or “If I Ain’t Got You.” Don’t pick something too hard if there’s a chance you might mess it up. Don’t pick something too easy, because that’s not very impressive. If you are really good at hitting high notes, pick a song with a lot of high notes. If you are a confident belter, choose a song that allows you to belt.  If you’re a great belter, try “I’m Not Going” from Dreamgirls.  Be aware of your weaknesses too. If you know that your vibrato is not that great, don’t pick a song that will force you to display that. Showcase your strengths and make your weaknesses invisible.
Watch the show. Listen to music. Be aware of your singing range. Pick a song that showcases who you are. Choose a song that proves your strengths.