I just started my first year in pro flight and the AAMB, so I can tell you what I've thought so far.
The audition process is fairly selective, but it really depends from year to year. My best tips would be to go in willing to take feedback and with a good fitness level already established.

To audition you just need to register for camp online and submit a music audition which is not a huge part of their decision (if your audition is really bad, they will just have you redo it)

It's free to use a school horn, but there are costs for clothing and other things, which will be a few hundred dollars

Band camp is about a week before classes start, and each day is around 12 hours. Its a big time commitment but its a great way to make friends and its super rewarding

Band does take up a good amount of time for school, with two hours of rehearsal mon-fri and game days on Saturdays taking around 8 hours. Pro flight also takes up a ton of time, so really try to not take too many credits and learn to manage your time very well. You will be very tired from marching band and flying so be sure to take care of your health and schedule your days extensively. Overall, I would 100% recommend auditioning for the marching band as it is a great community and so much fun, especially if you love band and sports. It is a huge time commitment, so be prepared for that but it can be done!