What is the best thing to do before sitting down to write a first draft screenplay?

Think about your characters. A lot! Who are they? What do they want? What are they afraid of? Get to know them like real people. Write character bios (for your sub characters too) and just riff for a page or two. Get personal. Make up stuff and start discovering their backstories. You don't need to know everything about them yet, but the more you understand who they are (and how they tick internally) the better off you're going to be when you sit down and start writing your first draft.

Oh, you already know who your characters are and want to get started? Cool. Then go ahead and check out:

The New Spec Rules for a First Draft Screenplay!