Your first step is to come up with a premise, around which your script revolves. The premise should be the driving force behind every event in the story, according to the American Film Institute. It's what gives the plot focus and a sense of forward-moving urgency. The premise doesn't have to be complicated. Depending upon whom you ask, a film's basic plot should be able to be expressed in anywhere from 15 to 25 words.
Even in very plot-driven movies, strong characters are what sucks an audience in and won't let them go. Try to think of a great movie that didn't have an amazing character or two in it. It's hard. And remember that they don't have to be nice people. Travis Bickle of "Taxi Driver" comes to mind, as does, Hannibal Lector, in "Silence of the Lambs." The secret is, for better or worse, the characters must be emotionally invested in the events transpiring in the story.
Screenwriting author and guru, Syd Field, makes use of a paradigm many successful movies follow. This paradigm is broken into three acts and often referred to as three-act structure. Act one is the first quarter of the screenplay. It introduces the main character and inciting incident for the story. Act two comprises about half the story and is concerned with introducing a series of obstacles to the main character, eventually bringing him to his lowest point. Act three contains the moment of maximum tension, or climax, between the main character and who or what stands in his way, then wraps everything up. Reversals, called plot points, separate the acts.
Hollywood studio readers are very picky about screenplay format, going so far as to simply reject any story that doesn't arrive with a standardized look. The easiest way to make sure your script is properly formatted is to use one of the major scriptwriting software packages out there. Movie Magic Screenwriter is one. Final Draft is another. Some basic tips are to always use a plain cover, white paper, and don't forget to three-hole punch the paper, but only use two copper brads, top and bottom, to fasten it together.