There's an interesting post by AJ McDonald over on the Lulu blog. He gives a thorough explanation of how to create an Epub from scratch. All of the information is correct, and his post is easy to understand. You should go read his post if you want to learn the basic technical details of the Epub format.
But if you want to make an Epub then I wouldn't follow his instructions. His method is far too complicated, and requires far too much work. Instead, I would download calibre and do the following:
- drag the DOC/HTML/PDF into calibre
- tell calibre to convert the file to Epub
- set the metadata, and add the cover image