More than any other form of writing the screenplay demands structure. A three act structure basically means a beginning, a middle and an end. Your first act should set up your characters and their situation.
You should set up your conflict as early as possible. In your second act your Hero makes efforts to resolve the Conflict, and may even believe he has succeeded, but ultimately the Complication thwarts him. In the third act our man finally resolves the Conflict and Complication.
Within this simplistic formula resides the basis of every screenplay.
Your art, as the writer, lies in using this formula to create interesting and compelling characters, a dynamic plot and sub-plot, gripping dialogue and stunning action. Sounds easy, doesn’t it?