What Makes a Story a Movie?

There's no set of rules for that, despite what script consultants might tell you (for a fee). For example, a book has some potential for a screenplay if it is an insider's story on a timely event. However, you're selling something that won't be on the screen for a couple of years at least, so the issue has to be fairly evergreen. A law case that embodies a controversy might be a candidate. Then you should learn about what's involved in securing underlying life-story rights. Otherwise, you might as well try to sell the news in the local paper.

As a story, a screenplay must: 1) have something important at stake (life or death, end of the world, social crisis, etc.), 2) have a main character with whom the audience can identify on some level, and 3) must require the character to grow and develop to resolve the crisis posed by #1. Stories that end in failure or disappointment are almost impossible to sell, despite some notable exceptions.

There are thousands of books on the subject of screenwriting, but the one I think that most nearly addresses this question is Save the Cat! by (the late, great) Blake Snyder and its sequel, Save the Cat! Goes to the Movies.

If you have such a book, it can be a mistake (or a waste of time) to write a screenplay yourself. Go to http://www.literarymarketplace.com and search agents who rep movie rights for books, and query them based on the criteria they give in their profiles or on their agency websites. Obviously the ones who accept e-queries will get you the quickest results and you can query them all concurrently. You'll know in a few days whether you have have something to sell.

If instead you think you have a passion for screenwriting, that is a whole different thing, all about your knack for storytelling, and long road.