I’ll approach this question from the “How do I get started?” angle. First, read a lot of books. Every one of my author friends is an avid reader. Decide which kind of books you like to read the most (for me it’s fantasy and sci fi) and concentrate on reading lots of those kinds of books. You’ll not only be doing this for fun, but also to learn different ways to put a book together—the right and wrong ways to tell a story.
Once you are well-versed in the type of book you’d like to write, start writing. By that I mean, stop coming up with excuses not to write (“What if it’s terrible?” “I just don’t have the time right now.”) If your first draft is “terrible,” you can fix it later. You do have the time, you just have to organize your schedule better. Write for an hour instead of vegging out in front of the tube, for example. Set a daily goal of a couple hundred words and do your best to stick to it. You’ll be surprised at how quickly it adds up.