Read the novel several times. Take detailed notes about dialogue, descriptions, characters and sequence of events, anything that will be useful when it's time to write the screenplay. Develop an outline.
Determine the story arc. Decide what to keep and what must be cut from the novel to fit into a screenplay format. Try to retain only the major plot and a sub-plot or two, if they're essential to the story.
Adapt an existing character or create a new one to have someone play off the main character so the main character's thoughts can be heard. Many times in a novel the protagonist's words are presented in thoughts as in a first-person narrative. Convert dialogue from these thoughts.
"Show, don't tell." This is an old screenwriting aphorism that applies most when adapting a novel to a screenplay. Movies are visual; novels are not. Therefore, you may need to be descriptive beyond what's written in the novel.
Consider a "based upon" screenplay rather than a straight adaptation. Use only the novel's settings, characters and story idea. This gives you a bit more leeway.