You must choose a subject for your book and determine what kind of audience you're writing it for. Unless you want to write a book solely for yourself, you need to market it to a specific reader. If you're writing a cookbook, decide if it's for dilettantes or people serious about cooking. If it's a piece of fiction, ask yourself what kind of fiction it will be -- mystery, adventure, romance, literary fiction -- and who would read it as a result. It needn't be too formal, but publishers will ask who the audience is, and you'll need to have an answer for them.
Once you have a basic idea, write up an outline for your book: an overview of the plot (or the general structure if you're writing nonfiction), two or three paragraphs on each of the main characters and a list of the expected chapters. You can make changes to the outline as you proceed, but it provides a basic road map to follow as you begin your work.
If you want the book to get published, you need a publishing house to accept your work, and you may want to obtain such an acceptance before you begin the hard task of writing. Write up a formal proposal and send it to publishing companies who may be interested in it. If you intend to self-publish (which is now very easy thanks to online publishing services), you can skip this step.
Set a schedule for writing your book and stick to it. Writing is an arduous process and, without a firm schedule, it's easy to become distracted. Whether you write every day or once a week, make sure you meet the schedule. As you get into the habit of writing, it will become easier to keep at it.
With all the preliminaries out of the way, it's time to write the book. Get the entire text down on the page before going back for rewrites. Every book goes through multiple drafts, so don't worry about whether or not everything is absolutely right the first time. The key is to get everything down; once you have that, you can go through and rework it to your heart's content.
Take a break once you finish the first draft: anywhere from a few days to a few weeks. It will help you clear your mind and regain the objectivity you will need to undertake the revisions.
After a little time away from the text, go back through the book and revise it to clean up any rough patches, bad grammar or awkward segments. Repeat this step as many times as necessary; you should give the book at least two or three full revisions. If you have someone you trust, let them read through the draft and provide constructive criticism. If you have a publisher, they may assign an editor to help with the process.
When the book looks as good as you can make it, finalize the text and print out a copy, then send it to your publishers or printers for final printing.