Write an outline of your plot, covering the structure of the story from beginning to end. Organize it by chapter and be sure to include all of the key events in your story. Writing it down doesn't mean you can't change it as you go along, and indeed, the novel may go through extensive revisions during the writing process. Having the outline gives you a sense of the workload, as well as a good notion of where the story is going.
Compile descriptions of all of your characters: a few paragraphs describing their history, personality, key motivations and physical appearance. As with the plot outline, they can undergo revisions as the writing proceeds , but with the descriptions as notes to refer to, you can keep track over their part in your novel and ensure that you keep their personalities consistent when you write.
Prepare a spot for writing, somewhere in your home specifically dedicated to the task. It should be quiet and comfortable, away from central areas like the living room where other members of the household may pass in and out. Keep it free of distractions---no television or video games, though you may want a radio or CD player to listen to music while you write. A quiet window view with a tree or something similar can help with the writing process as well.
Set a schedule for writing: a set period of time during the week dedicated solely to working on your novel. It can be every Saturday morning, an hour a day after you get up, weekday evenings when you get home from work, or whatever works for you. The only criteria is that it be a regular schedule---at least once a week---and that you won't need to break it for other commitments. Writing is a habit; a regular schedule helps you get into it and keep you from stopping when it gets rough.
Keep writing once you start. Don't focus too much on edits or changes, at least early on. Get as much as possible down on the page and try to write at least a little every time you sit down. Writing can't always be regulated: you might write 4,000 words in a single go, then turn around and struggle to get 300 words out on the page. Just make sure you get at least a little bit down every session.