moment you make the conscious choice and decision to write a screenplay,
the first question is--how to write a movie script. You probably
arrived at this decision after finding some information on
screenwriting. Another could be you are a fan of movies.
Whatever your motivation may
be, one first place where you can learn how to write a screenplay is by
simply going to watch movies. This first step shows you what movies
are being made and where your own idea for a screenplay would fit in this
"big picture" of what's out there. Also, by watching how a
movie is structured, this will give you how to structure your
screenplay. A great movie to watch for its details and how they are
put together is Whale
Rider or read this article on How To Watch
Movies As A Director, Actor, Screenwriter.
As far as the technicalities
of screenplay format and the like, you can find more information about
screenwriting software here.