To be a professional stage manager: do as much theatre as you can in school and/or community theatre--knowing all aspects of theatre is very important. Stage manage in school if you can. Maybe read THE STAGE MANAGEMENT HANDBOOK by Daniel A. Ionazzi--I haven't, so I can't verify if it is good, but it should give you ideas.
Intern or get a position as a Production Assistant at an Equity regional theatre. Training under stage management as a Production Assistant is amazing way to learn how to be a stage manager. Eventually, you will be offered the chance to become an Assistant Stage Manager and join Equity doing so. With experience and hard work, you will continue to reach your goals.
Stage Managers are very detailed oriented and logistically minded. These are invaluable traits. :)