Newbie question on making-editing ebooks

Hello, would someone kindly direct me to a web site that is similar to an ebooks for dummies? 
 

 I use, but am not an expert, in Calibre and Sigil, to edit existing ebooks I download. I have basic beginner questions, like: Why did I have to convert an ebook to a Calibre ebook, in order to add my own ebook cover jpg? Also, is there a basic for dummies way to code existing ebooks I download. I don't understand the ugly way folk's style the ebooks. Why all the Capital letters? Lazy? How do I break a subhead into two lines, but not two subheads?
 

 Any help would be appreciated.  John R