Getting started can be the most difficult part of the writing process. Set aside some time to make an outline of what your book will be about. Just write down your ideas and do not worry what order they are in. Keep a notebook with you at all times so you can jot down ideas as they come to you. Then put it aside for a day or two.
When you come back to your notes, choose just one of the ideas that you wrote down and expand upon it. Write for fifteen minutes and just let the words come out naturally. This may be more comfortable for you at your computer, but if you are used to writing by hand, do it that way. You don’t want to have to think about anything except for what you are writing. Write as much as you can BEFORE you go back and edit. This style of writing is called stream of consciousness and can be very effective in getting yourself to write every day.
The eBook writing course that I have used over and over is one from Jim Edwards and Joe Vitale called the 7 Day Ebook. I highly recommend it.