Study your genre. Rent 10 of the most successful films of the kind you seek to write and take notes while you watch them. Note how soon in the film we meet the hero, when the antagonist is introduced, what the biggest challenges the hero encounters are, what he seeks and how he attains or does not attain his goal in each of the films. Ask yourself how you could push each of these factors further---what screenplay guru Adam Levenberg calls "added value." Make your villain more fearsome, the odds more difficult, potential rewards if the hero succeeds even more tantalizing and the doom of failure more terrible. Show the reader a likable character who struggles against all-too-human flaws (lack of self-confidence, vanity, greed) and somehow comes to grips with these shortcomings. The reader, and viewers, will identify with your hero, root for him to win and ultimately become emotionally engaged with your screenplay and the film that is made from it.
Write a single sentence that sums up who your hero is and what challenges he faces to reach his goal. For instance, "George is a talented auto mechanic who dreams of one day driving in the Indianapolis 500, even though he's unknown in the race car circuit."
Write another sentence that describes the people or things that stand in the way of George attaining his goal, like, "Simon, George's overbearing boss, will not give George time off from work to compete in races that might get George the recognition he needs to become an Indy driver."
Write personal histories of all of your major and supporting characters: Where and when they were born, where they went to school, their likes and dislikes, hopes and dreams and the things that give them problems. Your protagonist (main character) may have a grumpy boss but an understanding wife. He may be short of money and must decide whether to enter the big race or put money aside for his child's education.
Compose a story that puts your protagonist in a quandary. Perhaps he can achieve the goal he's always aspired to, but to do that he must make a difficult choice, such as whether he must put his family first before his racing career, or perhaps, whether to be loyal to an old friend who has helped him advance his career.
Write the ending to your story briefly on an index card and post the card on a bulletin board. Then, work backward from the ending and write each scene that leads to the previous one you wrote. One you have established your story's structure, begin writing in a screenwriting word-processing program and flesh out your story.