Go see theatre. Small theatres, where the ticket prices are low, there are few seats in the audience, and the cast is non-union.
Once you've seen a show that you liked, once you've been inside a theatre and thought to yourself "I'd like to work here, this looks like fun" then find out from their program or their website who is in charge of hiring stage managers. Assuming you've stage managed a couple of shows in college, you can ask that person if they're looking for SMs and send in your resume.
If you didn't actually stage manage with your college theatre, then instead of asking for a job, offer to volunteer backstage. Then, do everything they ask of you to the best of your ability with a smile on your face and let it be known that you're interested in stage managing. One of these small theatres will give you your first job and then you're on your way!
That's all how I started, anyway; more or less.