Tips for auditioning for American Idol? Help appreciated.?

Alright, I'll be auditioning in a month and need to pick a song. I
have an alto(low female) vocal range. I'm best at singing pop,
country, or Christian. I'm best at low and middle range notes, and ok
with most high notes. I need to pick a song, so if something comes to
mind please share.

Also, would it be a bad idea to sing a Christian song? Although, it
has universal lyrics that could fit anything. I was thinking It's Your
Life by Francesca Battistelli.

Any general tips are also greatly appreciated for example: how to
practice, what to do while I wait, what to wear, etc.

By the way, I'm 15 if that makes any difference.

Thank y'all, God Bless.