I’ve had a lot of experience with this over the years, and I’d like to echo what SG said. Getting involved in the scene is the first step to becoming a manager. Simply by going to shows with the presence of mind to know that you want to work in the music industry, take time to observe.

Pick out the people who are working at a show and see what they’re up to. You can usually tell because many people will just be hanging out with their friends, but the working people will be focused on something a lot of the time. Those are the things you’d be focused on as a manager.

I’d start with a smaller band to get some practice personally. Take a percentage sure, but really try to learn as much as you can.

Also, if you aren’t already used to sending and receiving lots of emails. Get used to it.