Format your screenplay and write it correctly. Use scriptwriting software to do this. Scripts are written in specific format with specific elements. You need scene headings, action, character cues and dialogue. Scriptwriting software formats your script to industry standard with the "Tab" and "Enter" keys only, allowing you to focus on the specifics of writing. Final Draft and Screenwriter are commonly used script formatters. For free options, try Celtx. Scripped is a Web-based scriptwriting program that can also be downloaded for free. Any of these programs will provide an adequate framework to facilitate your scriptwriting..
Write back story and stage directions (the non-dialogue elements of your script) in third person, present tense. Include only what you can see and hear on the movie screen.
Wrong: She pulled a gun and thought about shooting him.
Two things are wrong with the above example. It's written in past tense. Also, how will a movie viewer know the character is thinking about shooting anyone?
Here's a better version: She pulls a gun and glares at him. Her finger tightens on the trigger. She breathes deeply and relaxes.
Create a protagonist (main character) with a goal. The story of your character pursuing this goal and the obstacles in his way will drive the script. The goal should be concrete (not something abstract, such as "to be happy"). An object, money, to free a nation, to win the heart of someone -- these are all concrete goals that could make an exciting script.
Create an antagonist who will stand in the way of your protagonist reaching her goal. The antagonist should be well rounded and as set on stopping your main character from achieving her goal as your main character is on achieving it. Your antagonist should be worthy of your protagonist.
Break your story down into three strong acts. Scriptwriting isn't an exact science, but you can follow a few specifics to create a perfect format for your story. Each page of your script represents about a minute of screen time. Place your inciting incident about 10 minutes (pages) into the script. This event sets your character off on his journey to achieve his goal. The first act should be about 30 pages long and should end with rising action (the action that makes the stakes of the story higher).
Place obstacles in your protagonist's way over the course of the next 60 pages. This is your second act. Each obstacle should gradually heighten the tension and make it unclear whether or not your protagonist will accomplish what she has set out to do. Your character should be reaching her lowest point near the end of this act.
Bring your character back to life by the first 10 pages of Act III. Act III lasts for about 30 pages, giving you a total of 120 pages, which is about a 2-hour movie. In Act III, your protagonist overcomes his obstacles, defeats the antagonist and achieves (or doesn't achieve) his goal. This act also often includes denouement (tying up of loose ends) after the climax.