Read as many mystery novels as you can. Devour mysteries by many authors, from Agatha Christie to current best-selling mystery writers. You won't try to copy their styles, of course, but you can get ideas to spark your own writing and learn some common mystery plot devices for you novel.
Brainstorm characters and ideas before you write. Your protagonist could be a detective, coroner or just a person with a regular everyday job who solves mysteries and crimes on the side. Research fields of interest to you. Create a villain and secondary characters to flesh out the cast list.
Outline your novel. You may toss the entire pre-set notion of your plot at some point, but for now, get an orderly list of situations on paper. Determine the tone of your book (funny, serious, nostalgic) and decide whether you want to write in first or third person.
Organize your workspace. Ideally, your office will be in a separate room in your house, shielded from daily noise and activity. Have all the books, papers and folders you'll need for research close at hand. Create a file in your computer's "My Documents" folder for each draft of your book.
Set a schedule. The only way to finish a mystery novel is to dedicate yourself to working on it everyday, or at least on a regular weekly schedule. Set a daily or weekly goal and stick to it. You can write a certain number of pages per week or write for a certain amount of time.
Learn about weapons, poisons and police work. "A Writer's Guide to Poisons" by Serita Stevens, the FBI's home page, the Forensic Science Society website and the National Law Journal will help you create realistic scenarios for your book.
Write and rewrite. Remember to end each chapter with a "cliffhanger" to keep the reader turning pages. Include conflict between characters and occasional false leads to keep the story interesting.