I started writing a book once. I actually got 400 pages in and just stopped.
Anyway, one piece of advice I have for you is just keep going! Don't worry about making every page and chapter perfect. Worry about that later. Just get the content out there and revise it later. People get bogged down in the minutia and they never end up finishing as a result.
Write about a topic you are really passionate about, not just something you think you should write about. For example, I remember an anecdote about a politician or something that really wanted to write a book. Like 5 times he tried but could never finish writing a book about politics. He was at his editor asking for advice about how to finish a book. The editor noticed something in his pocket and said, "What is that?"
"Oh, it's a list of beers. I'm really into beer tasting. These are some I'm planning to take some notes on later."
The editor said, "There's your book." Within a month he finished a book on beer tasting.