I spent a lot of years as a magazine writer. In the same way that your article for a magazine needs a hook, so does your story. What is that interesting thing that makes your story different and therefore why should people read it? Are you looking at a subject in a new and different way? One example of this is the plethora of vampire stories out there. But as long as you are coming at the subject of vampires in a new way, you can still make a new and interesting story out of it. (Although if you want to write a screenplay or a TV series, I would advise you at this point not to write about vampires… Sure, the genre is popular, but even if you come up with a new take on it at this point, it’s kind of been done to death. Uh, no pun intended.)
Anyhow, back to you hook... Once you know what your hook is, I advise you to brainstorm scenes (even if this is a novel) that pay off the premise of your hook. For example, if your story is about a countrygirl who moves to the city for the first time (yes, this is an old, rote story, but bear with me), you would want to show scenes in which the ramifications of her moving would be shown and also how her country-ness is juxtaposed against the backdrop of the city and vice-versa. You get the picture. Just brainstorm as many combinations as you can think of that you can make into scenes and incorporate into your plot.
One of the ways that scenes for your screenplay or novel can be created is through your main character's fatal flaw. Once you decide what your character’s fatal flaw is, you can brainstorm a bunch of scenes in which we see her committing this “mistake”. Of course, if you abide by the tenets of story structure, we should be seeing some kind of a progression, meaning that, throughout the course of the story, your main character should be getting over her flaw, at least to some extent. Even if in the end you decide that she doesn’t finally conquer her “misbehavior,” then we need to have seen her grow and change throughout the course of the story.