The first step for a new author is to choose the genre and topic he wants to write about. Do you see yourself writing fiction or nonfiction? Are you interested in romance or science fiction? After you've determined the appropriate niche, you can focus on what the basic idea behind your book will be. If it's a nonfiction book you want to write, you should pick a topic you know something about and prepare to do even more research so you are writing from a position of authority on your subject.
A worthy method to employ after you have an idea about what you want to write is to outline your book. Start at the beginning and lay out the plot for a fiction novel or the course you want to take in a nonfiction work. You may find it easier to break your book down into chapters and briefly describe what you intend to write about in each. Once you have this basic layout down on paper--or on your computer if you prefer--you are ready to begin researching.
Research is an important process for any book, whether it be fiction or nonfiction. Obviously, the latter genre will require much more in order for you to include the most up-to-date information on your topic. The former, however, might require cracking the textbooks as well. If you are writing about a character who plays the violin, for example, yet you yourself do not play, you should listen to violin music, speak with a violinist, and learn as much as you can about the instrument in order to make your characters as vibrant as possible.
Once you know everything you need to know, it's time to sit down and write. Don't be concerned that every word you write is publishable, just continue to get your words down on paper. It's the next phase, editing, where you will be able to take out everything that doesn't work, rewrite, and make sure the plot moves along in a way that makes sense to the reader.
Editing and revision usually take the most time in the book-writing process in an effort to make a manuscript as perfect as possible. As you revise, ask yourself whether you've developed your characters fully (if it's a fiction piece). Make sure the plotline moves forward logically. Determine whether every section or scene is necessary, and taking out the extraneous material.
Once your book is as strong as possible, you might want to ask some friends, family or even acquaintances to read it through for you. There will always be errors, grammatical or otherwise, that authors can miss through the editing process because they are simply too close to the text. A few more pairs of eyes will be helpful in solidifying its completion.