Writing a screenplay is hard.  It's a combination of skill, craft, and art.  There is a true beauty in a finely crafter screenplay.  Take these steps if you want to try your hand at writing your first movie screenplay!

STEP ONE:  Write your "My story is about a ______, who wants ______."  

STEP TWO: Craft your Logline.  A logline is a pitch essentially.  You could think of it as a teaser or hook.  A good logline centers on the main character, their goal, and the obstacle.  Here's two examples:

1. Pulp Fiction (1994): The lives of two mob hitmen, a boxer, a gangster's wife, and a pair of diner bandits intertwine in four tales of violence and redemption.
2. Toy Story (1995): A cowboy doll is profoundly threatened and jealous when a new spaceman figure supplants him as top toy in a boy's room.

STEP THREE: Work out your Beat Sheet or Outline.  This is a 2-4 page bulleted spine of all the major plot points of the story.  Think of these as the events that hold the story together and drive it forward.  The significant events.  Now, write these and then work on the characterization of your protagonist and antagonist and within that you will add more to your story.

STEP FOUR:  Write your First Draft.  Your vomit draft.  Your shit draft.  Whatever, just write it and try not to have it be less than 90 pages and not more than 120 or so.  Again, not rules just guidelines.

Good luck and get writing!