What does a Script Supervisor do?
Wikipedia has a fairly decent description here.

I want to become a writer is being a Script Supervisor a good place to start?
No, I would recommend looking into becoming a Script Coordinator.

How do I join the union?
Each Local has different requirements, contact the one nearest you for details. See the “Unions” page to find the one nearest you. If you don’t see one for your area please email me.