read-Read-READ!

I have forgotten to mention the most essential thing for aspiring writers to do,

Read

allot, not just books but literal screenplays that are out there on the Internet 

or head for the library and check out some screenplays on books, its essential to read, 

books will provide you with plot and character ideas, also as to how exploit the plot and make it big and grandiose with suspense.
A screenplay will better guide you towards familiarising with its format and style.

one page on a screenplay = 1 minute of screen time. 

And such minor details that separate it from a book or play.

As you read more screenplays you will get to know the plot and where its introduced, how the characters speak to each other, how the screenwriter used the setting to convey an important part of the story, and such details that you will also need to learn. Such as how explaining what the characters are doing and experiencing is the most important factor for a screenplay that will appeal to readers looking for a great film. And such as that the dialog must be short and to the point, leading to other points or mini plots or consequences or challenges that he main character must meet in order to be successful, on most screenplays with out giving the story away. Most screenplays also utilize minor characters to pass thought mini plots, they will guide the main character, help him or just be a distraction towards the main plot conclusion.
Plus you will learn how to better format your screenplay, that always helps write faster. Good luck.