Outline the themes of your story. Every story has a theme, or idea, it tries to get across to the reader. Make sure you know exactly what yours is. Write it down and post it up in your workplace so the emotional core of your work remains consistent.
Plot out the story in a brief outline format. Every novelist does this differently; some prefer random notes scribbled on paper, while others organize each chapter painstakingly by writing out conflicts and plot lines in detail. Do whatever works best for you to help maintain your excitement for the story. Don't write about something you are unfamiliar with. Write only what you have experienced or what you have researched.
Create your characters. Make sure you know not only their physical characteristics, but what they like to eat, whether they pay their bills on time, how they act when confronted, how many girlfriends or boyfriends they have had, what's their relationship with their families, and so on. For a novel it's extremely important you know the characters in great detail, even if some of those details are never printed on the page. Characters must act consistently and believably throughout the story. It's also quite normal and often recommended that the protagonist is sympathetic and relatable to some degree.
Write it out. Try not to edit your work too much as you write. Instead focus on moving the story forward. Don't be afraid to make things hard for your protagonist. The harder the character's life is, the more the readers will want to know how the story turns out. Try not to always write down the first idea that comes to mind. Normally that idea is a cliche and you've read it many times before. Think hard on each plot twist and each decision made by your character and try to avoid writing the same thing you've seen before.
Rewrite. When the novel is finished, take a breather, marvel at what you've accomplished, and then get back at it. It's often said that "writing is rewriting." No first draft has ever been accepted by a publicist. Write problematic scenes as many times as necessary until they're perfect. You may have to rewrite the whole novel two or more times, but that effort will show in the final product.