The formats for ereaders and printed-on-demand print books are different.
The Kindle format is derived from the Mobipocket reader; it is mostly a subset of HTML packaged in a certain way. Most of the other ebook formats are based on EPUB, which is also HTML packaged in a different way. For details see
this wikipedia article.
CreateSpace and the other POD publishers mostly use the PDF format.
There are services you can use to turn a Word or other word processor format into the three formats. Some services will turn a PDF book into the .mobi and .epub format, but they are not perfect. Especially if your book has a complicated structure and content.
_________________________________________________
The following is the method I use. Others in this thread will doubtless use different methods or (more likely) variations of this method.
I use Word to write my books and shorter works. Since I use CreateSpace for my POD versions I set the page setup to the trade paperback size I prefer and format the content exactly the way I want it to look when converted to PDF. This includes headers, footers, page numbering, margins, line spacing, fonts, and so on.
I use Word styles for this. Which means I can change it to the submission format preferred by the editors of most trade publishers of books and magazines and thus by agents (double-spaced, etc.) simply by changing the style. But my gives me a feel for the pacing of my book once printed and the visual appeal of the page.
Then I create a PDF output format using Word and submit that to CS. The very limited typesetting abilities of Word suit my purposes. But if your book has a complex format, images, tables, decorative elements, and so on you probably should hire a professional typesetter to create the PDF. (You can buy the typesetting software and do it yourself but typesetting is both an art and a craft; few of us can do that and not create a blah or error-full mess.)
I also create an HTML output format, again using Word. That goes into a program called
Sigil, which creates an .epub format book. This goes to B&N and Apple.
I run the .epub through
Calibre to create the .mobi format book. This goes to Amazon. (You can create the Kindle format first and create an .epub from it, but there are technical and practical difficulties with that.)
_________________________________________________
The first time you do this the method you use (mine or others) the process may seem very complex and difficult. But each time it becomes easier. By the tenth time it may be almost automatic.
Along the way you will learn ways to ease the process. For instance, instead of creating the Word file as one huge file I create one for each chapter and one each for the cover, front matter like title page and disclaimers and dedication, foreword, afterword, and "other books by" list.