Enroll in writing classes. Sometimes your local college will offer a screenwriting class that teaches the basics. You can augment this with a course in creative writing. Feel free to ask about the instructor's credits in the screenwriting field. Consider a dedicated film school (see Resources below) if there's one close enough to you.
Sign up for online screenwriting classes. There are plenty of these available (see Resources below). Many of these websites offer classes from qualified instructors. If you're uncertain about the quality of the courses, call or email the company with your questions.
Attend a live seminar if you have the opportunity. Robert Mckee's "Story" seminar is a highly acclaimed seminar on story structure. The seminar is also included in its entirety in the book of the same name.
Read as many books on scriptwriting as you can. Keep a handy reference book or two at your side while you're writing your script. Despite its title, "Screenwriting for Dummies" is an excellent quick-reference book on all aspects of writing scripts. The "Elements of Style for Screenwriters" by Paul Argentini is another handy reference material for scriptwriters.
Read as many produced movie scripts as you can get your hands on. Many movie scripts are published in book form. You can also find free movie scripts on the Internet, made available for study purposes (see Resources below). There's hardly a better way to learn scriptwriting than by reviewing material already successfully produced.
Write every day. Continue to study the art of writing scripts, but write your own scripts while you do it. Writing is the best form of practice. You can use screenwriting software to help you with the repetitive task of formatting scene headings, transitions and other screenplay elements. Movie Magic Screenwriter and Final Draft are the major programs, but you can download Celtx free of charge and accomplish the same thing.