Set your story up within the first 10 pages. This typically involves introducing your main character (protagonist) and showing him in his ordinary circumstances (inciting incident). By the end of the first 10 pages, a situation should arise that threatens to push your protagonist into extraordinary circumstances and take him from his daily routine. A general rule of thumb is one minute of screen time per page of your script. This means that you want to introduce a call to action that will hook viewers in about the first 10 minutes of your film.
Write the next 20 pages of action with your protagonist pursuing the goal of his call to action. This may be trying to win someone's love, trying to rescue someone or save someone's life, or trying to find a hidden treasure. Whatever the goal, the next 20 pages will feature roadblocks that your main character has to overcome. At the end of the first 30 pages (based on a 120-page, or two-hour screenplay), your character should be in a situation that it appears she may not be able to overcome. The first 30 pages signify the end of Act One.
Write 60 pages detailing the situation your protagonist is in and his attempts to overcome the situation. This is main conflict section of your story and will take up a third of your story. These 60 pages should result in your protagonist at his lowest point in the story. The more conflict, the greater the interest in your story. Viewers will root for your character. By the end of the 60 pages, it should seem that all hope is gone for your main character. This is the second act of your screenplay.
Bring your character back to life in the last 30 pages and wrap up the story. These pages of your screenplay will have your protagonist rising to her greatest challenge (as indicated at the end of Act Two). This is Act Three, where your main character achieves her goals (and sometimes doesn't). This is also where the resolution of your story occurs (climax). The ending can be happy or sad, but the story question must be answered. What did your main character want, and did she get it?