Let's get started

The easiest way to begin seems to be basing your screenplay on a novel (this is debatable). However, here goes:

Go into the novel, explore it from a visual point of view, then find a way to maintain the integrity of the piece. Then look at what you can or cannot use. Basically, you have a subject to write about (as opposed to making up an original one yourself.) So if the subject is convertable, go for it.