Scope out your band director and tell him/her that you want to be involved in the colorguard. (S)he will probably be more than happy to hear that. If you go to a well-known high school with a big band, you may have to try out. If you're going to a small school with a little band (like mine), you will most likely not have to try out for the guard. Try to talk to the band director before the next season starts, not in the middle of the season or anything.

If you know a friend in guard, talk to them and tell them that you want to join. Most likely, they'll be happy to talk to the band director about it.

If your school doesn't have a colorguard to accompany the marching band, you do have the option of trying to start one. Talk to the band director and tell him/her that you're interested in starting a colorguard for the marching band. She or he will probably be VERY interested. You will need to really want to start a colorguard and you will need to work for it. You may not be able to start a guard this year though. Colorguards in general are expensive to keep up. The band director may not have the funds to start one right away. You may have to try again next year. But don't let that thought get in your way. If you really really want that colorguard, you'll get it!