There isn't a short answer to that one (but I'll try anyway). Entire books have been written about screenwriting, and how to get a job therein, so here's -

Step One: Read some of those books. The more the better, because no one has The One True Answer on either how to write or how to break in.

Step Two: Write. Write lots. Then write some more. It's like anything else: practice counts.

Step Three: Learn a trade, acquire an inheritance, win a lottery, or marry money - anything to keep you fed while you:

Step Four: Repeat Step Two until your stuff is good enough to get an agent to take you aboard.

Step Five: See Step Three.

Step Six: Bother your agent to get you in to pitch ideas to television shows until someone hires you.

Simple as that.