Read. Read as much as you can, in all subjects and genres, and not only novels or other fiction. You will want to be familiar with more tales and topics than the average person so you have a large store of information from which to draw story ideas. You also will want to be familiar with the storylines have come before yours. Too often, new writers inadvertently revisit well-known or clichéd ideas without bringing anything new to the tradition.
Study writing style. Practice writing dialogue and description in different ways, paying attention to how each style contributes to or detracts from the writing. Practice with other well-known writers' styles, but do not copy them. You want to find your own natural voice. Also become familiar with the rules of grammar, and when it is okay to stray outside their bounds. Practice creating images in your writing, evoking feelings and making an impact.
Think about what kind of novel you would like to write. Whether you are the type of writer who first plans the plot, characters or an abstract theme before moving on to the others, make sure you plan for all elements to harmonize and make sense together. Remember that stories are essentially characters reacting to conflict. Many a novel has begun with a "what if" question, such as "What if Character X was in Situation Y? How would Character X respond?"
Turn the relationship between the characters and their conflict into a full plot. You may prefer not to plan and to let the plot unfold organically as you write, but be prepared to go in with a red pen later. Stick to the overall traditional plot structure of conflict to complications, rising action, and lastly climax to falling action. Be sure to make the plot interesting or exciting enough for people to want to go along for the ride. You do not have to give up the artistic aspect of writing or pander, but a successful novel will be able to be both art and entertainment.
Build the world that your characters live in. Make sure that the setting, too, suits your plot and characters, and that it adds something unique or recognizable to the story. Focus on creating aspects of the world that draw attention to thematic issues or characterization qualities in your story.
Research. The world-building stage should let you know exactly how much you will need to do, depending on how familiar you are with the place and time you have set your novel in. Every writer should do some amount of research. Even if you are writing a story that takes place in your hometown in the present day, you should still go out into the streets and scope out locations, talk to people, read about town history and analyze exactly what gives the town its flavor. Do the same if your story takes place in another location. Writers of historical fiction will have to be extremely knowledgeable about their settings, and may wish to do research themselves and consult the appropriate historians for information they cannot learn from reading the books in their local libraries. Fantasy writers are not off the hook either. Since they are building whole civilizations, they must know about how real civilizations came about and how those events affected culture.
Write. Write as much as you can, as often as you can, until you have worked through a good chunk of the novel. If you encounter problems early on, stop and evaluate whether there needs to be a change at the planning level. You may, once you start writing, find that your story wants to go in a different direction from your plan. Let it, and see what you come up with. Do not be afraid to make changes.
Revise what you have written. Be hard on yourself. New writers often make the mistake of assuming that a first draft is the purest version of the story, when it really is only a practice version. You may make several drafts and hundreds of revisions before the novel is ready for others to read it, so do not show it around before it is completely finished.
Write the final draft and polish it completely, so the story and style are as professional as possible. If you plan to publish, format the draft as a manuscript and start sending out query letters to literary agents you think would be interested in your novel. Be ready to confront the business end of writing and sell yourself as best you can.