It starts with either the concept or a scene.

The title should be memorable, showy and encapsulate the conflict within the script.  So for a TV Channel boss to sack a lead actor purely on the basis of personal differences, we have COLD SPITE.

You should write about what you’re interested in.  What you want to see in a play, film or read about in a book.  You may get inspiration from reality, you may get inspiration from your imagination.

Sometimes an idea about one scene fills in the rest of the blanks.

Other times it starts with the title, which can be a phrase.  Eg: CLOAK AND DAGGER, NOTHING TO LOSE, THE PHOENIX RISES.

However it starts, let it.