Welcome. You'll find a lot of good bits of wisdom on this forum. And some really awesome writers.
There are a number of online resources to get you started. One is Playwriting 101 (http://www.playwriting101.com/). Another is Richard Toscan's website The Playwriting Seminars. The first version is gone, but you might want to consider purchasing the newer e-book (which can be read on your computer if you download Amazon's free e-reader).