Writing a book...do I have what it takes?

Writing a book seems to be on everyone's "To Do" list, but something that most people never do. Sure, talking about writing a book sounds like a great idea. But actually sitting down and committing to it? Thats an entirely different story. Writing is something that has always come fairly easy to me, and unlike many of my peers, I enjoy doing. And, just like everyone else, writing a best-seller is on my "To Do" list. It's always been a thought that was floating around in my head, but lately I've been thinking very seriously about it. I just have one problem- I have no idea where to start!