Get a book on graphic design basics from your library, or look for a similar tutorial site. I doubt there's anything specific to book covers, but the general principles are the same whatever you design.
For ebooks, the main thing to remember is they need to work at postage stamp size as well as full size:
So use a simple image and large text with a lot of contrast. If you use Windows, you can check for small size clarity by looking at it in thumbnail directory display.