Play the game. Few become a successful baseball manager at any level without having played the game. Unlike broadcasters and general managers, the conventional wisdom is that managers need to have had a playing career. It doesn't have to be long or good (many major league managers never got out of the minors during their playing days) but you should have some background in being a player.
Catch. Catching is your best option as a player if you want to eventually become a baseball team manager. A great many successful professional managers started out as catchers because of their role in handling pitchers, setting defenses and generally running the game.
Know the game. Just playing isn't enough; you have to study the nuances of every aspect of baseball in order to become a manager. You must know the rules as well (f not better) than any umpire, and understand the pros and cons of every strategic situation possible.
Learn to manage people. Successful baseball team managers tell you that what happens on the field is only a small portion of their overall effort. You must learn how to get the most out of your team, how to motivate different players in different ways and how to handle the egos of everyone you manage.
Work your way up. Few major league managers begin their careers in the big leagues; most start as MLB coaches or minor league managers after their playing days. If you have your sights set more modestly on a job such as high school or college coach the same rules apply. Become an assistant coach or run a youth baseball team to prove that you're worthy of moving up.
Network. There are not many baseball manager jobs open at any level, so it pays to get to know the people who do the hiring. Attend meetings such as the Baseball Winter Meetings or your local baseball association where you're likely to make contacts. A general manager is more likely to hire someone he knows than to rely simply on a resume and application.