The main purpose of a book cover is to sell a book! Book covers are a marketing tool, so the right look is essential. Its design should inform, as well as attract, the reader.
To compete with other self-published books on the market, good cover design is paramount. It requires eye-catching imagery; should be genre specific; use a good choice of colour and contrast; have legible typography, with a well-written blurb (the text on the back cover).
I recommend using the services of a professional graphic designer. Although there are free-to-use cover design tools available online, these are limited in their scope.
(If an ISBN is allocated to the book, it will usually appear on the lower-right of the back cover, usually with a bar code.)