how do you publish a book?

For a while now i been thinking about writing a book. But, every time i go to start i make an excuse like it will be to hard to keep track of or Even if i wrote it then what do i do. I don't really know how to start either, should i do it one a computer or should i hand write it? I am not the best with spelling and grammar, how do i go about doing that. Anyone ever right a book? how did you go about it?