'I'm what you call a pressure-cooker writer. I give myself ten days to two weeks to write a first draft because it's too terrifying to have an open amount of space. I make myself start on a given date and then I have a first draft.'-- Naomi Wallace
Go through the section on Subtext before you start.
Go through the section on Plots before you start.
http://www.vcu.edu/arts/playwriting/