Honestly, it's all about connections: you either start with them, or you make them on your own by, generally, living where you want to work. If you start, you're lucky. You may know a friend who knows the wife of a stage manager, and can do some intern work. You may have a friend from college who became a director, and tag along with him for a while. You can do tons of things through connections, as that's what show business is all about. And yes, Broadway most definitely falls under show business.