The first thing to do is to seek education. It will not be a bad idea if you take some formal education in stage acting. This will help you in knowing how the whole process works. Also, if you have management education, that will be a bonus since a stage manager has to be able to manage people well. You may consider a degree in one of the two fields since it will help you in the longer term.
Hands on Training
Once you have education in the field and are aware of the basic working, you should get some hands on training as well. You can take a dig at acting or simply apply for an internship in the behind the scenes of a stage. This experience will be extremely helpful for you in learning the various nuances that are associated with the craft and will reflect well at your resume as well.
Keep on improving your skills with time. As the time goes on, your resume should grow too and you can move up the ranks of stage management. Make sure that you learn each and every craft within the trade well so that you can eventually move to the highest level.
Develop the Right Attitude
Along with gaining experience and getting promoted, building the right attitude is also very important. You need to be able to manage well and in a manner that commands respects from others and offers the same to them. Make sure that you are able to control matters when problems take place.
Whenever there is a job posting for a stage manager at a community stage, do apply. Keep an eye and you should be able to get there in a reasonable amount of time.