Get to know the medium of a play by reading and watching plays written by great playwrights.
Choose to write about that which you know. Choose scenarios, characters and themes of which you are familiar.
Develop your main character's basic statistics, including name, age, appearance, etc. Next, describe your character's personality. Then develop the details of your character's life, what he/she wears, likes, dislikes, habits, language, religious beliefs and employment. Also, create a past history for your character. By giving your character depth, you will avoid stereotypes.
Develop other characters you will include in your play. Explain their connection to the main character and to each other. Describe their basic statistics, personality and past history.
Plan the plot of your play. Your plot should have an introduction, rising action, climax, falling action and an ending. Decide on the location and interaction between characters in each scene. There should be an element of conflict present through every moment of the play, from start to finish. As you are outlining the plot, also describe what each character wants in each scene. Establish the mood and themes of the play. Make sure they are consistent and make sense throughout.
Practice writing dialogue between the characters before you start writing the play. A suggestion for developing dialogue is to have actors improvise a scene using your characters and play scenarios; write down what develops. Also, you could find a partner and take turns writing lines as if each were a specific character, again using the scenarios from the plot outline you developed.
Begin to write, starting with the first scene; writing a bit at a time. Imagine the play being performed as you write. Remember the limitations of a play, like how much time an actor will need to make costume changes, for example. Revise as you write by reading a scene again and again to make sure it flows and is error free.
Adjust your planned structure as you write. Often you will begin to write a play and the direction will change. Let characters take on a life of their own and go in the directions they need to go. Adjust your outline accordingly to accommodate the creative process.
As you write, keep in mind the following: avoid monologues, avoid narrators, avoid mundane scenarios and dialogue, avoid too many characters, pare down set changes to a minimum, and avoid the need for special effects. Also, remember to have action in each scene rather than just having the characters talk about what they want to do or what they have done. Allow your characters to "be in the moment" and let the drama unfold.
Review your play upon completing the first draft of your script. Make sure the characters are consistent and well-developed and that the plot is interesting and makes sense.
Host a read-through of your script. Use feedback from the read-through to edit the script.
Have your play performed. Be part of the production process as much as possible. Attend rehearsals if you can. Watch the final production. Use what you observe --- and any feedback from the cast and crew --- to revise the script again.