The first rule of writing has become somewhat cliche, but authors ignore it at their own peril: write what you read. This has also been described as writing what you love, but since most people don't read books they don't like, the two basically go hand in hand. For example, if you read nothing but mysteries, it is unlikely that you would be inclined to write a romance or a western, and less likely that you would ever finish it. The genre you choose must interest you, because you will spend a great deal of time writing it. What you most like to read is a good indication of what you would most want to write, and what you would be best at, since you are familiar with the form and style of that particular genre.
Because writing is a craft, you should start with some good reference materials. Besides a dictionary and a thesaurus, the most important book you can own to guide you through the many rules of grammar and writing is "Elements of Style" by William Strunk and E.B. White. This book is so much a part of the culture of writing that most of the time it is simply referred to as "Strunk and White." It's the one most people never opened in college, but for the serious writer, no single book will be more helpful, especially when you begin the editing process.
Two additional books that are very helpful are "Self-Editing for Fiction Writers" by Renni Browne and Dave King and "Writing Fiction" by the Gotham Writer's Workshop. "Self Editing for Fiction Writers" is by far the best book on the least-favorite part of the writing process. Editing your own work is extremely difficult, but the advice these two professional editors give will make your editing more effective, if not more fun. "Writing Fiction" packs a lot of great information into its 300 pages. Each chapter tackles a specific aspect of fiction writing (plot, character, etc), and each is written by published authors who are members of the Gotham Writer's Workshop, New York's acclaimed creative writing school. There are points within each chapter at which you are given exercises that allow you immediately put what you've just learned into practice.
You should start with an outline, a timeline, and character sketches. An outline is simply a tool to help you keep your story on track, and having a timeline before you start will save hours of correcting inconsistencies during the revision process. Write a character sketch with a full biography for every major character and for as many minor ones as possible. You need to know far more about your characters and their lives than you will ever tell your readers. For them to be three-dimensional, you need to understand everything you can about your characters.
When taking on a project the size of a novel, attack it in small pieces. Sitting down with the idea of writing a 100,000 word, 300-page book is a daunting task even for an experienced novelist; it can be overwhelming for a novice. Look at it in terms of words per day or pages per day. The average double-spaced page contains 250 words, so 4 pages per day will give you 1000 words. At that pace, you would finish in just over three months.
When writing the first draft, do not edit as you write. There will be plenty of time to edit when you do the first revision. Consider this more of a rough draft than even a first draft. Write fast, and put down whatever comes to mind, while keeping the story on track using your outline and timeline. If you allow yourself to become bogged down in editing one page or getting one sentence exactly write during the rough draft, you will probably never finish.
You also need to set a definite deadline for yourself. One of the biggest deterrents to finishing a novel is the belief that you will finish it "someday." As the Credence Clearwater Revival song said: "someday never comes." Don't wait for someday: seize today. Set a deadline that is realistic, but that will also challenge you. The 1000 words per day mentioned above are an achievable goal for most people.
Finally, have a comfortable place to write, and set a time to write every day. These two aspects go hand in hand, because if you don't have a place to write that is organized and away free of distractions, you won't keep the appointment with yourself to write daily. For some people, the library or a coffeehouse work best, while others prefer a quiet room in the house. Wherever it is, make that the place you always go. As for the time of day, each person is different. Some write best in the morning, while others wouldn't dream of starting before 10 p.m.; this is often dictated by the demands of work and family. Whatever time works best, make an appointment with yourself to write at that same time every day.
While there are a host of rules writers can follow, the ones outlines above are both practical and achievable. Following these steps can make the process of writing your first novel less intimidating and more productive, but in the end, actually doing it is up to you. The journey is often difficult, but the difficulties are definitely worth it.