Some writers never get started because they think they have to have it all figured out before they sit down to actually write.   Well I am here to tell you, you don't!  You don't need to have it all figured out, but you do need a few things absolutely resolved in your mind before you write.   As a matter of fact, I think you can over-plot and convolute your mind and your story, which can bog you down.  I've never been a note card person (not that there is anything wrong with that), it just wasn't my "screenplay starter." 

I think part of the joy of screenwriting is letting your characters evolve as you go and allow them to take YOU the writer into those dark or enlighten corners you never even anticipated!   If you over plot and over think your characters you could become to rigid in your writing and never really free them completely. 

So, with that in mind here are the "non-negotiables" I have to have before I start writing:

The First thing I start off with is character and situation.   I have to have interesting and well thought out main characters (Protagonist/Antagonist) AND place them into a situation that is dramatic.  This can be as simple as the location or setting of the story, but usually the situation involves a goal or question that has to be answered.  Think outside the box and always try to push the boundaries!  Sometimes I may not even have my Antagonist clearly fleshed out in my mind when I start, but I absolutely have to have the Protagonist and the situation s/he faces down pat.

The Second thing that I usually start with and absolutely have to know before I start writing is the ending and what happens, why it happens, and what does it resolve?   Your endings are buried in your first act and how the protagonist resolves the central conflict not only ends the plot but reveals the final act of characterization.  How and why your character does what s/he has to do is very important.

The Third thing is the plot, and I'm not so much concerned with knowing the organization of events when I start, but instead knowing the most significant and important Plot Points.  This usually should involve some sort of twist or surprise as well. Remember major plot events take the story in new and unexpected directions, those aren't "plot twists."  

As I have mentioned in previous articles, as a professional writer you are often asked by executives/producers to create treatments and beat sheets before writing and with those you really get a chance to flesh out your characters and story BUT you don't have to do that to start writing in my opinion if you know the above three important "things."

Again, this is just my thought process if you use a different method and it works remember, if it ain't broke don't fix it.