Yay, a tread about marching band. I've been in my high school school marching band for 3 years, and will be the woodwind captain for my 4th and last year of it. Man, I'll miss doing it after I graduate.
Now, about instruments, although I'm decidedly not the best musician, I can give some pointers. For starters, pick something you think you may like playing. This may sound obvious, but some people pick the "cool" instruments because they think they are "cool" (dumbasses). Most of the time this results in them hating their instrument.
Or you can do what I did and never play the same instrument year to year. I just stayed within the clarinet group from 7th-10th, played bass/alto/regular clarinet, and then this year switched to tenor sax. Saxes are quite awesome in my opinion, and it was much easier to play than the clarinet for me. Which brings me to my next point: if you really, really don't like the instrument you chose, or suck at it, switch to something else. It doesn't have to be radical, as most band people I know can go from their "main" instrument to anything else in that group (trumpt -> tuba) with almost no difficulty. I suck at playing regular clarinet, but am decent/good on a bass clarinet, and pretty good on a tenor sax. Just try instruments to you find what works.