Don't try to write a book in places where you conduct other tasks. Instead, set aside a portion of your home devoted exclusively to writing. A study or spare bedroom works fine, but any nook and cranny will do. It should have a door that you can close--and when you close that door, people shouldn't knock on it except during an emergency. Keep the area free of distractions such as TVs or video games (although some writers like to have a radio or MP3 player to listen to music). Make sure it's comfortable, a place where you can sit for an extended length of time while you are writing. Don't deviate from the space once you have established it. Use it whenever you plan to write; it will get you in the proper mindset more easily.
There will always be excuses not to write, just like there will always be excuses not to exercise. Some are better than others, but if you want to get your book written, you need to make time for it. The best way is to establish a schedule and stick to it. Set a time of the day or week to write, and when you start writing, work until you reach a set word count. Ideally, you should write every day, even if it is just a few hundred words. If you can't, try to write several times a week and use specific dates for writing (every Friday, for example, or every weekday). That gets you into the habit of writing, which allows you to power through reluctance and difficulty to complete the work.
Writing a book doesn't just mean sitting down and typing away. That tends to produce meandering shaggy dogs that require extensive editing to correct. Prepare for writing by devising an outline, covering the basic structure of the book, major events and pertinent points. You can deviate from it if your writing takes you in a different direction, but a preexisting structure helps to focus your writing and give your book a sense of pacing. Furthermore, be ready to perform research on the topic at hand. If you are writing a nonfiction book, research is extremely important, and you should double check all your stated facts. If you are writing fiction, you can still benefit from research, allowing you to speak about chosen settings or circumstances with authority.