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A screenplay is a blueprint for a movie. Besides telling the story, it also serves as a guide to the many professionals(directors, producers, actors, technical people) who will work together to create the final movie. Each person involved needs to be able to know at a glance what they have to do. For this reason, screenplay formatting is very strict. If an actor wants to know how much dialogue he has, he looks for his character's name in the middle of the page. If a producer wants to estimate a budget, he checks the page count and the number of "slug lines," a heading that indicates the beginning of a scene. A standardized script format makes everyone's jobs easier.
One screenplay page in Courier 12pt font translates to one minute of screen time. The standard length of a script is 100-120 pages. There are many screenwriting books available where you can learn more about formatting, and a Google search on "screenplay format" will turn up additional resources. The best way to learn formatting, however, is to start reading produced scripts. You can either buy them or find them on the Internet for free.
Once you understand the basics of formatting, you can use screenwriting programs, such as Final Draft and Movie Magic, to simplify the task of formatting so you can concentrate on your script.