1. Give Some Thought to Plot.
Writing a novel can be a messy undertaking. The editing process will go easier if you devote time to plot in the beginning. For some writers, this means an outline; others work with index cards, putting a different scene on each one. Still others only have a conflict and a general idea of where they plan to end up before diving in. If you've been writing for a while, you already know how your brain works and what kind of structure it needs in order to complete big projects. If you're just starting out, then this may be something you'll learn about your writing process as you revise your first novel.
2. Get a First Draft Down.
Though it is a good idea to test your idea out on other writers, resist getting feedback on the writing itself at this stage. Focus on getting the complete story down on paper instead. If you have trouble with writer's block or tend to let projects stall, NaNoWriMo might be helpful. Other writers maintain a regular schedule and spread the writing out over a longer period of time. Still others enroll in novel classes, which provide weekly deadlines and community.
3. Be Prepared to Revise.At a reading for his first book a few years ago, novelist Dominic Smith commented that the one thing he wasn't prepared for in writing a novel was the amount of work between first draft and published book. In one way, this is heartening. However inspired you might feel while writing it, the first draft will probably be bad. It will be clunky, disorganized, and confusing. Entire chapters will drag. The dialogue will be unconvincing and wooden. Rest assured that it's this way for everyone. And like writers everywhere, you just have to roll up your sleeves and get to work rewriting it.
4. Solicit Feedback.When you think it's time to start contacting agents, get feedback from writers you trust. Don't be surprised if they send you back to your desk for another draft. Address any large structural problems first, and then go through the book scene by scene. Anytime you have a question about whether something is working, stop and see what you could do to make it better. Don't just hope the reader won't notice. If you want your book to be good, revise with your most intelligent, most thoughtful reader in mind.
5. Put It Aside.
If you find yourself banging up against the same problems with every draft, it may be time to work on something else for awhile. Sixteen years elapsed between the first drat of Jane Austen's Pride and Prejudice and the published version, for instance. Katherine Anne Porter likewise took years on some of her most famous stories. If you find yourself losing your way, go back to the fun parts of writing. Create something new; read for fun. With each new project you take on and each book you read, you'll learn new lessons. When you come back to the novel — and you will come back — you'll see it with more experienced eyes.