Practice and prepare. Maybe ask your choir teacher if they can spend a few minutes after school with you to go over the solo and give you a few pointers. Talk to the other girls in the choir who might be auditioning and see how they are preparing. Keep hydrated and don't let your nerves get to you. And remember its not the end of the world if you don't get the solo but if you do get it sing your heart out little darling!
As long as you know the words I think you should totally try out!! I know that I've regretted a couple of solo opportunities and I think that trying and not getting the part is better than not trying at all! Of course it's really nerve racking for anyone to try out for a solo but youll feel soo good about yourself if you try!! I hope you gain the courage to try and I totally hope things work out! I Also hope I helped you a bit :)