Like in the business it doesn't matter what you know, it's who you know.
Best advice is to submerge yourself in the new york city scene, PA a bit, get in good with Broadway SM's and go from there. I think if you ask 20 Broadway SM's how to get to be a stage manager on Broadway, you will get 20 different answers.
Work had, schmooze and network.
Best of luck - lot's of people want to make it to Broadway.