Outline your plot. Yes, some writers just start typing and end up with a tight, cohesive piece of work. But if you really want your novel to flow and not leave out critical steps along the way, create a simple plot outline. It doesn't have to be elaborate; it's just for your own use, but it will give you a plan to help write the novel.
Describe your characters. Jot down the main characteristics of your main and a few of your secondary characters. Make sure that they are believable, real people and not dependent on stereotypes.
Write the first chapter. Start writing. Often, the most difficult part of writing a novel is actually setting pen to paper (or fingers to computer keys). It doesn't have to be perfect in the first draft. Just writing something creates momentum that will get your novel started.
Keep writing. Create time in each day---or week---to write. Writing a novel is like any other job; you get better at it the more you practice it. Even writing a few hundred words a day will eventually create a novel.
Look to writers' communities and forums for support. Writing can often be a rather lonely and solitary endeavor. Look to online and neighborhood writers' groups for support, encouragement and ideas.