Write, pitch, repeat. Working as a runner might help you network and understand the industry, there are various mentorship schemes, but ultimately nobody gives a shit unless you've got a great script. You don't need permission from anyone to write a script and send it to a commissioner.

There's a mile-long queue of people who want to be a script writer, but a much more exclusive club of people who actually write and pitch scripts regularly. As the great Ronnie Coleman said, "everybody wants to be a body builder, but ain't nobody wanna lift no heavy-ass weight".