I have a manuscript written already, I wrote it shortly after my mom and sister died. Just write it, you can search online on how to type a manuscript (there are rules, believe it or not) and then just type away. Make sure you save a copy to disc, that way if your computer crashes you won't lose your work. When the time comes, you may have to publish it yourself. Many a good author had to self-publish first.
May I ask what kind of book you would like to write?