Obtain a thorough understanding of screenplay form and structure. Apply to screenwriting programs at film schools such as the University of Southern California School of Cinematic Arts and the Tisch School of the Arts at NYU. For a more affordable alternative, the University of California Los Angeles offers the UCLA Extension Writer's Program, which is taught online and confers certificates in feature film writing and television writing. You can also take screenwriting classes at community colleges. These programs provide an education in screenwriting and film history, and also give you the chance to network with industry luminaries.
Read books on screenwriting structure. There are a number of books by script analysts and working screenwriters that provide valuable information about the elements that make a good script. These books include "Making a Good Script Great" by Linda Seger, "Story" by Robert McKee, and "Screenwriting: The Sequence Approach" by Paul Joseph Gulino. Use these books as reference materials throughout your screenwriting career.
Read recently-sold scripts and classic scripts. Visit websites such as Simply Scripts and the Internet Movie Script Database, which provide hundreds of scripts you can read online or download as a PDF or in Word format. Studying classic scripts such as "Casablanca," "On The Waterfront," or recent Academy Award winners such as "No Country For Old Men," and "The Departed" will give you insight into the craft of creating memorable characters, writing expressive dialogue and the art of constructing tight plots. Recently-sold scripts will give you an understanding of what the marketplace is buying, and will let you compare your finished script against scripts that are in development or in production.
Outline every script you write. Though some writers ignore this step, an outline is important because it helps you build the correct structure for your script. Think of an outline as the frame and foundation of a house, and the actual writing of the script as all the things that fill the inside of a house. Outlines help you craft well-structured screenplays and give you the chance to resolve story issues in advance.
Develop a network of trusted colleagues to critique your work. Whether it is through a writing program, an agent or manager (if you have one) or an industry contact, you need people in the film business who will read your script and provide you with honest feedback.
Rewrite multiple times. A good screenwriter must understand that rewriting is the key to generating well-written screenplays. You may end up doing a dozen drafts before your script is ready to be put on the market. After the script is sold, the studio or producer will want more rewrites, as will the director and the stars who play your characters. Without the enthusiasm and diligence required to rewrite, you will not hone your skills and develop into a good screenwriter.