Welcome to MobileRead.
You may want to take a look at our
Workshop subforum, where many ebook makers (including a number professionals) hang out to discuss the trade.
as well as
calibre which has a forum
here and contains many helpful tools for ebooks. Including an editor, and inserting metadata like covers.
Also the standalone editor
Sigil.
Regarding the EPUB format: most computers don't recognize or support them, until you install one of the above tools or others (Adobe InDesign and iAuthor spring to mind).
Word of advice. If you have the original format for the book -- say, a Word document -- you will save yourself endless trouble. PDF doesn't convert well to anything, for numerous reasons discussed extensively in the workshop and elsewhere.