something to think about before you begin

Before you write your screenplay you should have a logline. A logline is a concise synopsis of your story. It's the it's about. The thing you say after the "what?" You know what I mean; you're having a conversation with someone, and out of the blue and apropos of nothing you interrupt with, "I'm writing a screenplay", and she says with bewilderment, "what?" Well, right after that when you continue with, "Yeah, I don't think it's been done before. It's about..." Right there. That bit. After "it's about". What you say next, that's the logline (the act of saying your logline out loud in public is called 'the pitch' or 'pitching'). Now, there are debates over its concision, but generally it must follow the pre-determined format of a something someone or something in a something situation must do something to someone or something to change someone or the something situation to something different than the someone or something before something else happens.

Once you've got that down, or something like it, you are then permitted to start.

Here are a couple of examples with corresponding scripts: