I’ve always wanted to write a book. Where do I start?

That’s one of the most common questions I get from folks. Without intending to be flippant, the answer is . . . write it down. You don’t have to start at the beginning. You don’t even have to have the whole idea flushed out. Just get something on paper (or in the computer). That’s the first step.
If you’ve always wanted to write a book, you probably have a topic that is itching to get out. Many writers start with what they know. Maybe it’s something that happened to you that you want others to learn from . . . or laugh at. Perhaps you’re an expert in a particular field, and you want to share your knowledge.
Even if the book you start isn’t the book you finish, it’s a place to start.
Get in the habit of writing. Some people swear by writing a few hours at the same time every day. I write once a week. Find a rhythm that’s comfortable for you and turn it into a habit. Don’t write in your “spare time.” If you’re anything like me, you don’t have any to spare. You always have demands that will pull you in several directions. Put writing on your calendar and allow yourself that time to be dedicated toward your dream of finishing a manuscript.