Qualifications
Just like musicians don't need a degree in musicology, band managers don't need any fancy degrees or diplomas. But they need to develop their business skills somehow. Some schools offer degree programs in music management, which is basically a hodge-podge of courses in business, recording, and entertainment law (source).
You might want to take some music courses too. You don't have to have the best singing voice in the world (you probably don't), but you do need to understand what you're listening to if you want to promote it.
If university degrees aren't your thing, you can jump right in if you happen to have a couple of friends who want to start a rock band. Or you can do what some people call "scouting," which is another word for "making friends with people who happen to exclusively be talented musicians."