First, in order to write books, you must first read books. This isn't optional. Without first being a reader, you have no perspective on the medium you're working in. How will you know if you have a unique angle if you're not aware of what angles are already out there? This is the same for screenplays. In order to write them, you have to not only watch movies, but read the scripts that created them. Lots of them. Read everything you can get your hands on, regardless of genre. Screenplays are much shorter than novels, but they are also much, much stricter in format and (generally) in formula. The limitations on how many characters, scenes, plot points, and pages a screenplay can have are much more concrete. It is also, generally, harder to get a screenplay fully realized than a manuscript, as publication is significantly less expensive than filming. Once you've decided on the medium you want to use, and you've done the necessary reading, that reading will inform you on how to move your characters from plot point to plot point. This is the way most creative people learn how to do what we do--through example. You need to widely expose yourself to an art form in order to be able to produce it yourself.