If you are going to do a course or a degree, I'd suggest accountancy, commercial law, or an MBA (wary of the latter though, many of them are BS). You are then set up for a career in any business.
Bear in mind that the airline industry, for many reasons, is like no other, and as others have said, not something you can learn other than by practical experience.
When I've applied for jobs or contracts in the airline world, I have NEVER been asked for proof of any relevant formal qualifications or degrees, which given that I have none, is probably just as well. Ironically though, a couple of years ago I was approached by a business school for a part-time lecturing position in aviation management and practice and had to fill in a form giving details of academic qualifications etc. Left most of it blank, they sent it back for completion, I said I couldn't complete it because I had nothing to put in there. They had to get special exemption to employ me, even though it was they who had approached me and I didn't really even want the work.