Help me PLEASE! In a hurry?

Im 13 and graduating from my middle school coming up in May. The song must be school appropriate and yah. Uh........I want to play the guitar to it and sing. Im a girl, btw. Also, it can't be too high or too low. Ex: picture to burn-Taylor Swift (all other songs by her are fine except Our Song) and Kelly Clarkson (all songs). Also, how do I gain confidence and not SOUND nervous, even though I will be? its in front of the whole 8th grade, but first, i have to audition in front of my pod too, including the guy that i like!! ahhhhhhhh!! and we cant sing together either; it has to be solo. Thanks to all that help. Also, people keep saying that I sound like Taylor Swift, so yah.