When you decide to write an ebook and create a Mobipocket version of the book, the best place to start is with HTML. While you can convert Word documents and other file types to mobi, since Mobipocket is based off HTML, HTML is going to give you the cleanest files.
The easiest way to convert your HTML to mobi is with the program Calibre. You can get the program at their website http://calibre-ebook.com/. You can get Calibre for Windows, Macintosh, and Linux. It's free and easy to use. Calibre is primarily an ebook reader, but you can also use it to create and edit ebook files.
Open Your HTML in Calibre
Once you have Calibre open, you should click on the “Add books” button and browse to the HTML file of your book. Calibre will open it calling it the format “ZIP” which it obvously isn't, but don't worry about that.
On the next page I'll show you how to convert your HTML to the Mobi format.