Books are judged by their covers. This is one truism (or "truthiness") that seems to always apply. If you have decided to self-publish, let me recommend that you take some time to study book covers. Look at covers that have caught your eye and covers that are appropriate for your genre. By this I mean BEST SELLING covers for novels in your genre. Some covers out there, especially among self-pubbed books, are very amateurish.
I chose to go a little outside my genre (thrillers), and incorporate some imagery and cover format that had worked to draw my eye. I have always been partial to a solid block of color behind the author's name/title of the book. Some cover artists can blend the type into a dark spot of the cover art, or a place where there is a solid color. This allows the title to "pop" and also allows the reader to delineate between the image and the title. The two should connect, by the way. The image should reflect in some way either the title or the theme of the book. Unless there is a murder at the picnic, a bucolic scene with a field of flowers, a bench with a family around it laughing does not a mystery cover make.
Covers are art. That is the way you must see it. This art can be as simple as type face over a solid color, photo images and type or illustration/painting with type, or any variation of those. The point of the art, the point of the cover, is to draw potential readers to pick up the book, to click on it and read more.
Now, I by no means am an expert at this, but I have a good eye. I have read thousands of books and have perused libraries and bookstores for three decades. I know what stands out. I have watched the trends. I notice when a book cover for a particular novel (i.e. Game of Thrones or The Stand) changes. I notice when a cover I find appealing, but is outside of my reading circle--romance, for example--catches my eye and then becomes popular. For sure, the popularity of novels is not dependent upon their covers, but on their contents. Some books succeed IN SPITE of their covers. The Stand was one of them, Stieg Larsson's Girl series is another.
The practical side of the cover is the design of it in preparation for print. Many of the Print-On-Demand publishers like Create Space and Lulu provide tools to make this task easier. But, just like many other areas of self-publishing, you must don another hat, develop another skill. If you don't have proficiency with Photoshop, Adobe Illustrator, Adobe InDesign or some other design studio software, I recommend that this task is one that you hire out to someone that does. A good graphic artist, a cover designer, or even a graphic design student can provide the PDF document or JPEG image you will need to provide your cover.
Another practical consideration you must keep in mind is the art/photography. I am not a photographer. I used to be a fair illustrator but have not honed those skills in decades. Find a photographer you like or subscribe to iStock photos or eShutter or some other online stock photo marketplace. You can get pictures there royalty free for less than a dollar. Or, if you like the personal touch, digital photography is an art form in itself. If you would like to marry that talent with your talent for writing, then that could be your best option.
The beauty of self-publishing is that YOU have the control. YOU make the decisions. However, it is also the burden you bear because, in the end, the success or failure is on you as well. You have to have good content, a good story told with craftmanship. You have to have an attractive cover that draws in readers. You also have to have a good marketing plan that puts your title, your name and your brand in front of as many people as possible.
NEXT UP: EDITING