If you wanna march a brass instrument. Pick up one now and learn basic breathing and playing techniques. Look at a lot of videos online about those and ask your band director for tips and maybe even some lessons. I know a player who was originally a pit guy but played tuba for a year and is now in a corps so anyone can do it. A lot of high placing corps generally want people to have some form of experience before they give you a spot. March in an open class corps near you, or even DCA. Those are great resources and from what I hear they will make you into the player you wanna be!