Choose a theme and storyline for the movie. It should involve something teens can relate to such as school, relationships, sports, being popular or being a geek. Decide if it will center on a specific group such as the cheerleaders, the jocks, the nerds or the rebels.
Pick a setting for the movie. High school is usually the best, but a beach during summer or a shopping mall is good. Or if you're ambitious, mix it up with different scenes set all over.
Add the underlying message of the movie. It usually deals with learning to love yourself and be happy with who you are. You may just end up teaching your teen audience something. Of course, your movie might not have a strong message at all--think "Dazed and Confused" which is more of a "day in the life" sort of story.
Add the basic elemental characters. This might include the popular but snobby girl, the hunky but dimwitted jock, the good looking guy who secretly has a heart, and the nerdy girl who is actually a babe under the horn-rimmed glasses and baggy clothes. Of course, it's best to stay away from stereotypical roles and create roles you've never seen in the movies before. Write what you know: Base some characters after your friends and other people you know.
Plot out the twists and climaxes to your story by drafting a loose outline. Starting from the end and going backward may work best, since the climax makes or breaks the story. Make sure you have a good plot twist, usually somewhere between the halfway and two-thirds point of the movie.
Flesh out the script. Then read it and rewrite it, adding whatever else you think is needed and removing what isn't. Repeat this process until you have the best story you think you can write.