Write an outline for your story. Think in terms of a thee-act structure. What is your inciting incident? This is the incident that sets your main character, or protagonist, in motion. It could be a tragic death or a loved one being kidnapped. It might be a secret introduced that needs to be figured out by your lead character before something drastic happens. This inciting incident is act one and will encompass about the first 30 pages of a 120-page screenplay. The final couple of pages in this act need to end on a high dramatic note.
Outline the second act of your story. This is the main thrust of the screenplay and where your protagonist is faced with challenges and obstacles in the pursuit of the story's end. This act encompasses about 60 pages in a 120-page screenplay. The final couple of pages in this act should end on a high, positive note, leaving the audience with an idea that your hero is going to achieve his or her story goal.
Write the last act of the screenplay, which encompasses about 30 pages, to leave your audience satisfied. It's time for your protagonist to succeed or fail, though you should have a very good reason for a hero's failure. Use these pages to tie up loose ends in the story and wind it down to a satisfying conclusion.
Follow the three-act structure without stressing about it. The number of pages per act are a rough estimate. The key is to set the rising and falling action at the appropriate places within the three-act structure.
Use script formatting software to write your screenplay. The elements of a screenplay have various margin settings that are necessary and repetitive. Dedicated screenwriting software does the formatting for you so you can focus on writing the story. Final Draft and Movie Magic Screenwriter are the movie industry standards, but you can use Celtx free of charge (see Resources below).