Before you launch into creative mode, you need to attend to a few practical matters. Check to see that no one else has already secured the rights to the work you wish to adapt. If it's available, you need to ask the author--the person who holds copyright--for permission to make a movie based on his novel or screenplay. If the author is deceased or the copyright has expired (after anywhere from 25 to 95 years) then the material is classified as "public domain" and you won't need to secure any rights.
Accept that there's no way to condense 200 or more pages into a two-hour movie without cutting out a lot of details. Your task as a screenwriter is to create a "faithful" representation of the work. Don't create a carbon copy, but rather capture the "spirit" or essence of the work on film. Get to know the material intimately. It will be up to you to decide which characters, events, themes and descriptions stay and which will go. Define your vision, and select the details that best communicate that vision. Remember, with film you are working in a different medium. Whereas a book conveys sights, sounds, smells, events and insights through words only, a film tells a story through moving pictures and sound. As American screenwriter John August says, "Adapting a book into a movie isn't a matter of feeding the pages into a projector." You are making a series of cinematic choices to convey your vision to viewers.
Decide if your screenplay works best as a feature-length film, a made-for-TV miniseries or another format. You also need to determine if you want to invite the author to read your work and edit or offer input during script creation. Have fun playing with the pieces of the story. You can add, delete and tamper with time, order of events and other setting, character, plot and conflict nuances. There is no shortage of movie-making techniques and special effects to work with. If you do not have a film-school background, you want to research the craft so you have all of the tools at your fingertips. The "Screenwriter's Bible" by David Trottier is a good start.