If you have an idea for a movie script/screenplay and little to no writing experience, translating your idea into written form can seem a bit daunting. The first step is to fully form your idea by creating a plot outline (i.e. basic story bullet points), character breakdowns (descriptions of each main character), and a character development analysis (a description of how each character will change or grow throughout the story). Once this is done, you can begin writing what's called a film or script treatment.
A treatment is a short (usually only a few pages), broad description of a movie or script idea that covers key details pertaining to the story and the potential production. It is meant to persuade the reader that such a production/movie deserves to be made. A treatments starts with a logline; a 1-2 sentence description of the movie that usually includes a genre (i.e. action-adventure, romantic comedy, etc) and the target audience. Remember, this is the first thing the reader will see in your treatment, so make sure its compelling and tantalizing.
The second part of the treatment is the character description. A good character description involves a one paragraph overview of the story's main characters, often including references that identify these characters with known actors or movie roles.
Next, you'll need write a plot outline. This outline includes several 1-2 sentence summaries of key scenes. Keep it short and exciting as to entice the reader. Finally, your treatment should end with a concluding paragraph that explains why your movie will be a commercial success and why target audiences will want to see it. In general, keep it short. Producers read treatments on a daily basis. To hold their attention, you need to make sure its concise and arouses interest. It should be written in present tense and showcase the idea in an engaging manner.
After your treatment is completed, it would be wise to get your idea registered with the WGA (the Writer's Guild of America) or the Library of Congress. This just ensures that if someone steals your idea, you have some legal leverage. Once that's taken care of, it's time to shop around your idea. Send the treatment to as many producers, writers, and production companies as you can. It's a numbers game, so get your treatment out there. After a while, if there seems to be no interest in your idea, you may want to consider writing the script yourself or hiring out a writer. Some producers prefer to have a fully written script along with a treatment before considering a project.
An idea for a movie script/screenplay is just that, an idea. It doesn't become tangible until you can describe what that idea looks and feels like. That's why a treatment is so helpful. It gives the reader a real sense of the story and it's characters so that they can picture it themselves. After all, the reader knows nothing about your idea until you describe it to them. Get them interested. Draw them in. Show them why you think your idea is interesting and worth watching on the big screen.