First, learn how to format a script. No one will look at a script if it’s not formatted correctly or if there are typos and grammatical errors all over it. You have to make it look professional.
Correct formatting can also help you come to grips with a story’s structure. Every story has a beginning, middle, and end, and so does every screenplay. The book I used for formatting guidelines in school was Syd Field’s Screenplay. It’s a great book, and it’s easy to use, but there are a lot of others on the market—go out there and find one that works for you. Find published professional screenplays and copy their formatting. If you’re following a professional format, you know that the first ten pages have to grab readers by the throat and keep them reading. The next thing they know, they’re at page twenty, and then page ninety, and then they’re saying, “My gosh, that was excellent.”
It’s also important to realize that screenplays are visual in a way that’s very different from novels. Every sentence in a screenplay has to be something that happens visually. The words have to jump off the page and—as I believe I heard a producer at the Oscars once say—dance from your mind all the way to your heart. Novice writers often spend too much time on the way characters develop internally. And that’s important, but you can’t linger there. You have to translate that development into specifics and visuals, cut out some of the middle stuff, and get to the point of the story. You have to complicate things and then get to the ending without all of the fluff that can creep in along the way.