There may have been a magical occasion where a play jumped fully formed from a playwright's imagination onto the page and up onto the stage without any rewrites or drastic changes, but we don't know about it. Most, if not all, playwrights need to revise and rework their ideas before a play is finished. One way playwrights do this is by attending workshops. "Workshopping" gives playwrights a chance to share their ideas with other playwrights, hear how scenes sound when acted out, and explore different ways of bringing their visions to the stage. Take a look at a few different workshops (http://www.playwrights.ca/ and http://theatre.missouri.edu/mpw/index.htm).

So you want to be a playwright . . . first you need an idea. Take a look at this idea generator (http://www.seventhsanctum.com/generate.php?Genname=quickstory). Try to write a scene based on the random idea the generator spits at you. Who knows? Maybe you'll take it all the way to Broadway!