Read this, re-read this, go to a seminar if you can. It will save you time, headaches and make your story much better.
This book has helped me immensely in terms of gaining clarity, focus, and a much more compelling and entertaining story. Many of us start the writing process with a character, a scene (that we get attached to), and then try and string it together with some rambling story. This book with help you avoid that trap.
Both movies goers and producers make many of their decisions based on a one or two line summary of a project. STC shows you how to develop a killer logline (one line summary). If you do this and cannot get anyone interested in your idea, why would you continue to write until you can make it interesting? If you have a weak logline, believe me, the screenplay will not get better as you expand from one sentence to one hundred pages! STC gives great guidelines and a beat sheet to help avoid this. However, it is still your responsibility to come up with that great idea.
This is one of the best, most user friendly books in the genre that I have read. His second book is highly recommended also.