Because there is no definitive right or wrong way to do it, aspiring novelists can only hope for general guidelines on how to write a novel, rather than seeking out a step-by-step guide. Novel writing is, like any other type of writing, a craft that takes immense amounts of practice and critique, and starting small will almost always benefit the writer. Most of all, when seeking out advice on how to write a novel, be ready to hear a common answer that you may not like: be ready for rejection, and be ready to become adaptable and immune to it.
Novel writing is a practice in stamina. First, you must cook up a good topic. Then, you must develop characters. After all that is out of the way, you must stick with your story for weeks, months, or even years. But before any of that can happen, you must recognize the first step in the long list of steps when learning how to write a novel: gain a strong vocabulary, learn correct grammar, and practice writing well. The difference between a good writer and a mediocre one is the language, so be sure to find ways to practice using strong language, vivid words, varied sentences, and so forth. If you are out of practice, consider taking a college course or joining a writer's group where you can get critique from other writers.
This is where rejection comes in. You will often hear the word "no" as you learn how to write a novel, and as people honestly critique your writing. Do not take these criticisms personally; you can learn the most about your writing from negative feedback, such was what weaknesses you need to improve, what you can do better next time, and how you can better position yourself for success.
Once you are somewhat confident and comfortable with your basic writing skills, it's time to enter the writing community. Not only will other writers help you learn how to write a novel, but they will also serve as contacts in the publishing industry. A strong community of writers is vital to any aspiring novelist serious about learning and refining the craft. Consider going to writing conferences to start establishing contacts; if this is not an option, try joining online writing forums. Beware, however, of sharing your work online with people you do not know. Develop a relationship of trust with other writers first, then share your work for critique.
Organization will prove to be one of your strongest assets when learning how to write a novel. Regardless of your genre, your plot can easily become too intricate and complex for you to remember all the details at once. If it helps to make an outline of characters and plot points, do so. If it helps to simply take notes, that works well, too. In general, you will have to learn who you are as a writer before you can write a good novel. Learn your own nuances, needs, and wants, so that you can make yourself better prepared to tackle a long-term and complex story.
These are only the very basic first steps of how to write a novel. It is a long and complex task that can ultimately be incredibly rewarding, but it will take persistence and patience on your part. If you are new to writing, try writing short stories first to test your hand at fiction. This is a good way to practice your writing, and it also gives you the opportunity to collect writing credits, which will become very important later on when you attempt to sell your completed novel.