Playwriting has parallels with screenwriting in that structure is very important, and you start by deciding on your theme. The theme is the deep subject that you are exploring, and once you can describe this in a single sentence you are on your way. Structure then becomes the framework on which you build your characters and your story.
Your characters then go off on a journey, usually an emotional journey, and should emerge as different people at the end. But as you take them (or as they take you) on this journey, you still need to be aware of structure. At the end of each scene, for example, a character should be faced with a decision to take as the scene closes. Even silence on stage can be a structural tool: it can give pattern and shape to your script and can convey emotional weight and tension.
These and so many other techniques are explored in Belona Greenwood’s book, together with sound advice on marketing your script.