How often have you had the thought: "I know so much about this topic, I could write a book about it!" But you've never written that book, have you? Too often we find outselves not acting on the great ideas we have, and when it comes to writing a book, more often than not we don't get past that initial great idea stage. But the truth is that it's not that hard to write a book! If you think you could or should write a book, then maybe instead of musing about how fascinating your book could be, you should get down to it and start writing.
How you start will depend on what sort of book you want to write. What will your book be about? Selecting a topic is a necessary first step. Then, decide what the purpose of your book will be. You can write a book for about as many different purposes as there are books in the world... but generally an author writes a book for one of a few usually purposes. Your goal may be to entertain, to inform, to convince, to instruct or to provoke an emotional response (shock, or sorrow, for example). But you should not even try to write a book until you know for certain what you are trying to accomplish.
Once you have your topic and your purpose, the planning stage begins. Writing a book requires a tremendous amount of planning, but it will be well worthwhile in the long term. It will result in a better-written book, and the part where you actually write the book will be that much easier, because the challenging legwork will be done. When you write a book, your pre-planning serves as a roadmap. Not having a plan is like driving in complete darkness. You just won't know where you're going!
To plan the book you're going to write, you must consider your purpose. What will be the best way to accomplish that purpose? You may need to do some serious research before you write a book. Research may include reading other published books, searching through magazine and newspaper archives, studying primary sources, and possibly contacting and interviewing experts. If your book is a personal memoir or work of fiction, you can write the book without too much research. But you must still consider how you can write a book which best communicates the ideas and has the effects you're hoping to have. A funny memoir should highlight the humor in your experiences; a tragic personal narrative should linger more on the difficult and trying times you've been through.
Once you've created an outline, go ahead and start writing. You'll find that if you just get started, you can write a book quickly and let the ideas flow. Before you know it you'll have a first draft of a manuscript. It's really not that hard to write a book. The trick, really, is to get started!