Complete your idea. Before you sit down to write your script, make sure you have a whole idea: a beginning, a middle and an end. Assemble your cast of characters and assign names before you begin. Feel free to give every character a name as a placeholder to come back to later.
Look at some other play scripts to have an idea of the format. Most plays follow a fairly similar format, but they do vary. Dialogue will always be written like this, however: HAMLET: To be, or not to be.
Map out your idea. Take notes. You can use file cards or a simple pad of paper, but map out your script and how you will progress from the beginning to the end. Most plays are performed in two acts: before and after the intermission. However, most plays do not simply have two acts. Shakespeare’s plays were all written in five acts. The act often signifies a passage of time or a change of setting. Each act is usually broken down into scenes, which are a conversation between characters. For simplicity, especially if you are just starting out, try to divide your script into two hour-long acts, divided into anywhere from 4 to 10 scenes.
Simplify and then simplify some more. Assuming you want to see your play performed, there are some things to keep in mind. The more scenery changes, the bigger the cast, the more complicated the effects, and so on, the less people will want to stage the show. It might be a great play, but if you write a show for a cast of 20, it will be a tough sell. You should shoot for ten cast members total, but some performers can play more than one role. Likewise, look for places in the script where you can have more than one scene in the same location to cut down on scene changes.
Write the script. Start with a scene you’re excited or passionate about. Write it simply, describing actions in between lines of dialogue. The most important thing is to get the words on paper and organize them later. Talk to yourself, act it out, do what you have to do to get excited about the script and start committing it to paper. Once you have a scene, write the one before it, or the one after it, or the next one involving the same characters. You shouldn't feel like you have to write it in order, so long as you keep writing.
Finalize the script. One basic format, from Shakespeare, is actions and stage movements are written in italics and entrances and exits are written in brackets [Hamlet enters]. Go back over the entire script and make sure you like the order. Use a critic’s eye to search for things that you like and want to work, but which may not. Finally, you should always do a reading of the script before it is performed. Some lines that look great on page never sound right when spoken, so take the time to hear what your whole play sounds like before you send it out to theatres or agents.