Pick something you are going to be able to perform to maturely. If you are wanting to do a more intense song, test yourself, it can be pretty hard not to smile if your friends are all screaming in front of you. Performing can be considered even more important than the work, you have to be prepared to perform to your song whether its happy, suspenseful, intense, sad, whatever.
If its your first time..maybe start with something easy to figure work out to. Something with definite high points and low points where you can easily put a piece of equiptment to and make definite transitions.
Make sure when you choose your music the judges will take you seriously. I dont know how many routines i saw last year at solo and ensemble that i wanted to just cry for the performers. The more serious you are, the more serious the judges will think you take guard and how much you really want to succeed through your routine. Of course, there may be some of you who just want to do this for fun and thats cool, but you can find a way to have fun WITHOUT doing a routine to Hit Me baby one more time or something :-D!
When you start to make up work really listen to the music and especially the lyrics. Make sure your work fits the mood of the song and everything matches with a good beat. Dont try to make up several eight counts and cram them into the song, do it an eight count at a time and if something doesnt work, change it, dont leave it there and hope that you will figure it out later. Pace yourself and dont save everything for one time...it makes your routine rushed and probably full of easy filler work.