Sorry I can't help with the mellophone player... but I may be able to help with some other stuff.
It's good you have a plan - because a goal without a plan is only a dream. Now that you have a good plan for it, it's definitely a goal.
First - Get in shape. Look up "First Running" (Google it) follow that training program (or another if you wish) to prepare for a 5K (run.) This will get you in shape. After that, do similar work outs (most long runs) just to stay in good cardio shape. This helps a lot in playing your horn. It helps your breathing (lung capacity) playing (support) and your overall physical wellness. Along with this, add other physical training. Such as sit ups and pushups. Nothing helps support like a 6-pack (dead serious) and nothing helps you keep your horn to the box at the end of your show in August (or November for marching band) like some push ups before then. That physical training will help you prepare both for drum corps and for your marching band.
Second - playing ability. The big things are: Tone, technical ability, sight reading, musicianship, practice ability. If you have these, you'll be able to whip any piece of music in no time. I'll send you my daily practice in a PM or an email sometime. But basically, do tone training, scale and articulation studies, sight reading, etudes and solos on a daily basis. Also, practice your in band class. Be sure to make it 'musical' read around the music. Figure, dynamics, articulations, etc. Get picky with it. Make every little bit perfect. In a corps, you'll practice your music that well, might as well get used to doing it now.
Third - never give up. You've gotta begin with the end in mind. Remember your goals, your plan, and everything you want to end up to be. This is the best motivation to keep on pushing.
Fourth - (probably should have been 3rd) Marching. You should strive to be the best marcher in your high school band. Learn how to teach others to march, because you learn a lot about your own bad habits when you teach somebody else how to. Learn how to separate your upper and lower half (so as not to 'hear' your steps in your sounds.) Learn horn carriage, marching style, etc. Everything involved in marching. Being a good marcher is as important as being a great player.
Speak with as many former corps members as you can. Of course speaking to vets of your dream corps would be the best - however, it'll be beneficial to talk to any vet at all. One great way to do this - dci.org. Somewhere on there - is an audition tip section. Read through some of those (made by vets) to get an idea of how to audition.
Seek private lessons. Both for playing and for marching. My trombone lessons range from $15 for a half hour (from a college student) to$ 40 and hour (the professor.) It may seem expensive - but they're worth every penny.
I'd say good luck, but luck is only the meeting of opportunity and preparation. Your opportunity will come, so come prepared. :)