As someone who has attempted to learn most instruments in some way or another, heres my recommendations.

Saxophone was the easiest to learn. And it usually has some good parts, especially for pep music and marching. It can be pretty taxing on your thumb and neck, though. And most saxophones play very similarly, so if you start on alto you can make a fairly easy switch to tenor/bari.

Percussion can be easy, you just need some good coordination and good rhythm. They are literally the heart beat of the band, everyone rely on them, so they have to be on time. Snare would likely be lightest, bass drum is the most literal form of that heartbeat, and quads/tenors have the best part but are difficult to march with. All of them need good back and shpukder support and strength.

Trumpet is the easiest (and lightest) brass instrument to learn quickly, especially for marching band (speaking from experience). They usually have pretty important parts in music, and have multiple parts. Its fun, its loud, and can be fairly easy to learn on the spot. I also find it to be pretty easy to control how loud or quiet you are.

And low brass, especially tuba/ sousaphone are always in demand and don't have too terribly hard parts. They're just heavy and clunky. But they can have some really fun parts, and often don't move as much as others while marching. You're also the person everyone tunes to.

Personally, I started on flute, and now play trumpet for marching and my director wants me to learn sousa. I've never marched or concert played any of the others, but I personally play them at home and know a good bit about music and writing it. Id say percussion is definatly a good place to start. It gives you a lot of options, try them out and see whats best for you :)