The first step is almost always the most frightening. This definitely rings true when it comes to writing a book. Since publishing my first novel I have had conversations with hundreds of people that say they'd like to write a book. The sad reality is that most of them will never accomplish this goal; not because they can't but because they're too afraid. When people ask me the secret to writing I almost always tell them that it's all about firsts: first word, first sentence, first paragraph, first page, first chapter. Simply start, and then just write one word at a time and watch what happens next.
I would never have imagined that I could write a 72k word novel, then an 87k word novel, but I did. My first goal was to start. Then I just kept writing as often as possible. Writing a book takes initiative, discipline, and determination. If you focus on those three things and you have a real passion for writing, you'll be able to write a book. Maybe it will be a bestseller, maybe not, but those of us who love writing the most write for one reason -- to tell a story that no one else on this earth can tell.