Take some time to read popular works in your chosen genre (young adult, science fiction, romance, mystery) before you begin your novel. You'll gain valuable knowledge about what works and what doesn't for contemporary readers and publishers -- after all, there's a reason those novels were selected for publication. Plus, you'll absorb a great deal of information you didn't expect -- rhythm, character arc, language, voice. Every good writer is a great reader to begin with, for just that reason.
Create a story outline that describes the beginning, middle and end of your story. This outline can be as detailed as you would like. Writers who prefer to let the story tell itself may have an outline that's only a paragraph long. Other writers may detail every chapter before they start their first draft. Both ways work equally well. The point is that every writer knows how the story begins, how it ends and how his characters get from point A to point B before he sets pen to paper. You should, too.
Write a brief biography of each of your main characters. Believable stories are driven by believable characters, so the more you can understand who your characters are -- where they came from, what happened to them before the story starts, their attitudes, how they react to different situations and emotions -- the better your story will be. Take time to get to know them.
Start writing and don't stop. Write your first draft fast and furious. Don't worry about grammar, don't worry about style, don't worry about anything but telling the story you want to tell. Your first draft will be terrible -- and that's okay. The point is to get it out of your system, because it is too easy to get caught up in editing and rewriting as you compose your novel. Doing so will slow your momentum and may cause you to abandon the story.
Return to your first draft and begin your second -- possibly the first of many new drafts. This is where the real work begins. Read through your first draft and start taking notes on what is good, what's bad, what makes sense, what doesn't, what needs to be added and what needs to be taken away. Ignore spelling and grammar mistakes as best you can. That's for your final draft. As you write and rewrite, you'll discover that your story gains greater depth, your characters become more alive and the story you wanted to tell in the first place starts becoming clearer and more concise.
Share your work with trusted colleagues. Try to forgo giving your book to friends who will likely tell you what they think you want to hear. Instead, find other writers (many cities and towns have writers groups) and book lovers who will share constructive criticism about your story. You want fresh eyes on your work, and you want someone who will highlight your plot holes, inconsistencies and any self-indulgent writing that you can probably do without.
Produce one final rewrite based on the constructive criticism you received earlier. Try to remove yourself as the book's writer and objectively review the remarks others made about your story. You're likely to find that they are right, and the work you'll do to repair those sections of your story is invaluable. Then pop some champagne, buy a new outfit, go for a 10-mile run or indulge in a movie marathon to cap off your final rewrite with a much-deserved celebration.