Marching bands tend to be huge, and organizing a group of people that big is no mean feat. So that means that in order to have a successful marching band fundraising experience, you'll need to start early and get yourself organized, so that you can feel confident and in control when it comes time to organize everyone else.
Having a meeting with the band teachers, and any other people in authority is a great idea. Discuss with them their hopes for marching band fundraising, what they need money for most, and what sort of time frame they're thinking of. Start writing things down, start filling in the equation. The next thing you need to know is exactly how many people will be participating. With fundraising it's the more the merrier, but that doesn't mean you should feel discouraged if you have a relatively small marching band.
There are good marching band fundraising ideas out there for all sizes of bands, and no matter how many people you have, you can make this work. It's up to you whether you want to meet with the band members themselves before making a final decision on which marching band fundraising idea to go with. Some organizers prefer to keep the decision making process small and simple, and then you can just present the idea to the students after it has already been finalized. Others like to include the band in the decision making process, perhaps selecting two or three "finalist" ideas, and letting students vote by ballot or by a show of hands.
If you have a particularly large band, it can be difficult to get everyone to agree on a marching band fundraising idea, but you can start by eliminating the least popular choice, and then voting again between the other two.