Qualifications
You need an acute understanding of the game, its rules, and how to execute offensive and defensive strategies. You must be able to teach the skills of the game to amazing athletes who might slip into bad habits. You must be fair, approachable, organized, and able to keep large groups of people happy when they're tired, stressed, and lonely.
No major league franchise will hire you without having confidence that you embody these characteristics. Getting a shot at being a major league team's manager is less likely than playing for a major league team. Most players hang up the cleats after high school, and the rest trade in their jerseys for suits and ties after four years in college.
The truth is most players that'll reach the big time get drafted before their nineteenth birthday—then have to spend years in the farm system (which, no, doesn't involve mooing; the farm system is all the minor league teams affiliated with the major league team).
And thank goodness for those farm teams—in addition to the players that hone their craft at the lower levels, all of those teams need managers, too. If you're wondering where you develop your coaching skills, it's not in your hometown's little league program. The managers of those teams usually go by the less formal title of "someone's dad."