Come up with an idea for a story that involves a ghost. It needn't be fully fleshed out or even complete; it just needs to be intriguing enough to get you interested and rich enough to create an entire novel.
Explore the notion of what ghosts are and how that plays into your story idea. Most ghost novels posit the spirit as a departed soul lingering on the mortal plane for reasons known only to itself. Some ghost novels portray the spirit as an otherworldly entity with no human origin, while others ascribe a theological implication to their ghosts. You'll need to develop an explanation and a back story for your ghost, even if those details appear in only the vaguest possible way.
Work your basic idea into a complete plot, entailing the appearance of the ghost, the havoc it wreaks, the goals it pursues and the normal people (typically your protagonists) who interact with it. Horror stories generally work best when they take place in a mundane or ordinary world--it helps the audience relate to the main characters, which enhances the fear--so provide settings and characters which remain as plausible as possible.
Write your ghost story with a focus on a fearful atmosphere, intended to evoke a chilling mood of suspense in your readers. Novels have an emotional rhythm which requires a certain "down time" between scares. Plan the spooky parts carefully so that they flow naturally into your story, and use descriptive passages, scary settings and scenarios which tie into the inherently terrifying nature of ghosts. Make sure you yourself can feel at least some of that fear; you can't scare other people if you aren't a little scared of the material yourself.
Enhance the sense of mystery behind the ghost in your story. Not only do intriguing unanswered questions help draw your reader through the novel, but the unknown is an inherently scary prospect and can contribute a great deal to your overall story. Determine how many details you should reveal, they way they change the story and the method (hopefully scary) by which the readers are made aware of them. Include a few unexpected twists in the mix as well, very few interesting novels unfold the way everyone expects.
Revise and edit your novel once it is written to improve its content. Few novels work best as a first draft: go through your text when it is complete to tighten the pacing, rework difficult passages, eliminate redundant or unnecessary wording and make sure the story flows as smoothly as it can. At some point allow an editor (or qualified friend) to make a few editorial passes over the novel as well. She may notice elements in need of improvement that you yourself may not.