Write a 1-page synopsis of your story. Outlining the play scene by scene is also a good idea, but begin with a brief synopsis. You might be asked to produce a synopsis later anyway, if you try to sell it. The synopsis will help you keep the story moving forward, by establishing a beginning, middle and end.
Write a story with a message. Powerful drama is often powerful because it has something to say, or a point to get across. Maybe your story can serve as a cautionary tale, or as inspiration for people to lead better lives in some way. Use the gift of writing and story telling to affect people’s lives. Think of the stories that have impacted you most; most likely, those stories had something to say about humanity.
Let the most exciting drama that develops be character based. Your characters must ring true if you want to move an audience in any way. If you have a solid story, but lack strong characters, the play will not be effective because it will feel false. The audience or reader may feel that the play’s situation is believable, but the characters experiencing the situation would never exist. Let the audience get to know the characters. Give them a relatively lengthy setup, so that when everything falls apart in their lives, the audience will relate to them, because they understand who these characters are.
Follow the standard play structure. This is not to say you absolutely must be formulaic in your approach to your play. You can stray if you feel the story calls for it, but if you are going to try to sell you play, agents and executives can be a little wary of such scripts. Be sure you have a strong setup, an introduction to the main characters, a problem that your main character or characters must contend with, the struggle to fix said problem, and a resolution. This is not to say you must write a happy ending, but that the story must simply be resolved in some way, happily or not (see Resources below for more specific structure ideas).
Invest in playwriting software. If you are going to try to sell the script, it is an absolute must that your story be formatted properly. No one will ever read your play if they don't know when a character is speaking, or where the stage direction is. Therefore purchase software, and let your computer format the play. Focus on the writing. Your play will not only have a better chance of being produced; it will be much easier to write (see Resources below).