There are many ways to create a document suitable for conversion to the more popular eBook reader formats. You can, in fact, use just about any Word or OpenOffice (or LibreOffice) document and get something that looks pretty good on the other site. Maybe not perfect, but pretty good. Another way is to create a simple HTML/XHTML document. It's easy if you just follow these simple steps.
The ePub (and Kindle) spec supports most of the valid HTML/XHTML tags which is akin to a disaster waiting to happen. Here's a tip. Keep it simple. It's less work and looks better. The simpler the HTML, the nicer the book will look. Seriously. The tags you need are . . . drum roll please.
<p> - for paragraphs.
<h1> - or any of the other sub-heading tags; it's best to use the <h1> tag for the title and all chapter headings.
<li> or <ol> - for any lists.
<b> and <i> for bold and italics.
Yes, that's all there is to it. Mind-bogglingly simple, huh?
Of couse, there are limits to this approach and you can get a lot more complex with different tags, tables of content, images, cover page graphics, and so on, but if you want a clean, easy to read document, this isn't a bad place to start.