What Exactly Is a Script?
A script is a document that outlines every aural, visual, behavioral, and lingual element required to tell a story. Why "outlines"? Because film is a highly collaborative medium and the director, cast, editor, and production crew will, based on your "outline", interpret your story their way when it is filmed. They may consult you, or they may not. Other writers may be brought in or you may be asked to re-write the entire thing. That's life, in the world of screenwriting. But because so many people are involved in the making of a film, a script must conform to standards that all involved parties understand and thus has a specific format or layout, margins, notation, and other conventions. This document is intended to overview the typical elements used screenplay writing.
It is crucial to remember that film is a VISUAL medium. You don't tell your audience your story, you SHOW them. You must learn to write a screenplay VISUALLY. Write what they will SEE and what they will HEAR. You might love your characters and know what they are thinking, but the discipline of screenplay writing is how to show it on a screen. When it happens, it may be just done with a look, often improvised on the movie set. So just write the pictures, sounds, and speeches, and leave the rest for the filmmakers.
What Makes Good Story?
Let's hazard a guess. The movies you loved most featured characters that swept you up, who captivated your emotions, got you involved. The audience viewing a movie not only wants to be interested in and care about the people they see on the screen, they want to be PASSIONATE about them, whether they like them or not. Great heroes and heroines inspire us; great villains make us want to jump into the screen!
There is always something at stake in a good movie. Not just something someone wants, something that must be acquired, no matter what the risk, as in Indiana Jones and the Raiders of the Lost Ark. Or something highly desired by as many main characters as possible, like the small black statue in The Maltese Falcon. Some times it can be an intangible thing, like the freedom of a people in Lawrence of Arabia or Gandhi. All these things drive the character's quest, even gives the hero superhuman strength. It can be something personal (romance) or for the good of all (saving the world from aliens) but it must be powerful and grow more desperate as the story unfolds.
There are always obstacles, which provide that catchword that actors love so much -- CONFLICT. This is the heart of drama. Someone wants something and people and things keep getting in the way of them achieving the goal. At times, the obstacles can be common to both the hero and villain, and the ultimate goal a laudable one for both parties, as in Jingle All The Way. In that film, Arnold Schwarzenegger and Sinbad battle to achieve the same goal--the acquisition of the last popular action figure for sale that Christmas season. Both of them have promised their son, and they must not fail. Conflict and obstacles can be physical or emotional. But they have to be in your story or you don't really have a story. In most good stories, the protagonist will also have an inner obstacle, some mental or even spiritual problem, that will be resolved by the time s/he reaches the outward, physical goal of the story. Some people call this inner demon a "ghost," while others call in a "wound."
You need a hook. That's a songwriting term that describes that thing that catches the public's attention. A popular Hollywood term is a "high concept." A better idea might be a simple "What if?" In Galaxy Quest, for example, the concept is "What if the washed-up actors from the crew of a cancelled but still popular sci-fi TV show are pressed into a real war in space by aliens who think the TV show broadcasts they received were documentaries?" A good enough "what if?" will set your script apart from the pack. It is why people will leave the comfort of their homes and plunk down their hard-earned bucks at the local cineplex.
Hollywood buys genres. Agents, managers, and producers are drawn to and specialize in specific genres so approaching them with something they can recognize is a good idea. Successful stories have a fresh face but are identifiable. You know what makes your idea unique, but can you describe it quickly to others? Is it a fast-paced thriller, romantic comedy, action adventure?
Scripts have to look a certain way. I can't stress this point enough. You must present your work like an insider. The sheer volume of submissions makes it so that if ANYTHING about your script looks strange it's headed for the circular file. If you don't know the game they won't play. The scriptwriter has to adhere to conventions covering everything from how many pages to what font (Courier 12 pitch in the U.S.), and that's just the beginning. I recommend you follow those rules, unless you're independently wealthy and plan to finance, produce, and direct your movie. Even then, however, the people you'll need to work with will be accustomed to standard formats.