Come up with a plot idea that will lend itself to a visual medium. For the purposes of this article, your project will be a 15-minute "short" as opposed to a two-hour feature film. The plot should revolve around something that is familiar to you, like your family, your friends, your school or even a memorable vacation. It can also be an adaptation of a favorite fairy tale, myth, or folk tale. Keep in mind that film is all about action. Therefore, a movie in which your characters move around a lot is going to be more interesting to your audience than one in which they sit at a table and just talk the whole time.
Identify the characters in your movie as well as when and where the action takes place. Since 15 minutes isn't very long to tell your story from start to finish, it's best to use fewer than six characters for this project and only one or two scenes. Decide whether the events transpire in the present day, the past or the future. Decide how long a time period will be represented -- an afternoon, a weekend or even a flashback/reincarnation that bridges two time periods.
Create a three-act outline for your movie before you start writing it. Since you want each section to contain an equal amount of content, divide the total length of your film by three. In this case, you'll have five minutes to spend on each of the three acts of your story. Because each page of a typed script is equal to one minute of screen time, you'll have five pages for each section. The first act introduces the main characters and establishes what the conflict is (two boys want to ask the same girl to a dance); the second act escalates the suspense (it looks like she's going to choose the sneaky boy over the honest one); and the third act stacks all of the odds against the hero (he loses his job and someone steals his car, making it impossible to ask her out). Just when all hope seems lost, the end of the movie reveals that the sneaky boy stole the car, the girl finds out, and tells the honest boy she thinks it would be more fun to go on an inexpensive picnic with him instead.
Study actual screenplays. Screenwriting is all about following an exact set of rules insofar as margins (1 inch on each side), font (12-pt. Courier New), paper (20# white bond), and the placement of elements such as character names, dialogue, master scenes, action and transitions. You can begin by downloading free samples of scripts from websites such as Simply Scripts, Drew's Script-O-Rama, and Script Crawler (see Resources).
Create a screenplay template in Word. (If you already have screenwriting software such as Final Draft loaded on your computer, you can skip this step because everything will be automatically placed for you.) If you're using Word, your options include downloading a template maker (http://www.online-communicator.com/spmacros.html); studying the script formatting chapters in screenwriting books; or creating your format manually by setting tabs at 2.5, 3, 3.5 and 6 inches. These tabs are respectively for the placement of dialogue, parentheticals (how a line is said), character name and transitions such as "Cut To" or "Fade to Black."
Familiarize yourself with screenplay terminology and abbreviations such as INT (interior), EXT (exterior), VO (voiceover), and CU (close-up). Find a complete list of these terms at Screenwriting.Info (http://www.screenwriting.info/glossary.php). Websites such as this will also advise you on which elements need to be capitalized and how to set up scenes such as intercuts (for instance, two people talking on the phone), and create transitions from one scene to the next.
Start your script in the most exciting place possible. Whether your story is 15 minutes or a full two hours, you need to hook your audience right away by throwing your character into the thick of a dilemma. Maybe your main character witnesses a crime, locks herself out of the house or is chased by an unknown party through a dark wood.
Ask your friends to read your dialogue out loud for you so that you can hear if it sounds natural or stilted. Often what reads just fine on paper will sound awkward, contrived or breathy when it's being spoken by a live actor.
Join or start a screenwriter;s critique group at your school. This will give you a forum to test your movie ideas on your peers as well as read each other's work and offer advice on how it can be improved.