Everyone knows the cliche saying "Don't judge a book by it's cover". I think a lot of us try to follow this saying, but most times that doesn't happen. We as readers do judge a book by it's cover. That's why it baffles me when authors put such hard work into their story but do very minimal for their covers.
Readers DO judge a book by its cover. If you're cover looks rushed or looks like very little thought or effort was put into it, readers will feel that way about your writing before they even read it. Think of your cover as a job interview. When you go for a job interview you want to dress the part because first impressions stick. (There's another cliche that reigns true). If you wear sweatpants your interviewer will feel that you're lazy or not really care about the job. You may actually care about the job and be hard-working but your interviewer already made up their mind about you.
The cover of your story is the first thing that readers look at. It has to catch their eye before they consider looking at the blurb. By having an interesting cover readers think "Hey what's that?" Your cover needs to be aesthetically pleasing and show elements of your story in the art work.
Another part of the cover that is important is the title. You title needs to be intriguing and say something about the story. Readers are drawn in by the title then the cover.
How do you make sure that you have a good cover for your book? Most publishing companies design your cover for you but if you are self-published you have to go through a graphic designer to get a cover. Many self-published authors I know use Createspace. A good idea is to get two covers made then poll your followers on social media on what cover they find more attractive. (Don't do this if it is too costly of course). Don't use pre-made covers. Most of the time pre-made covers are put onto other books. It is not good to have the same cover as another book since people identify books with their covers.
In addition, make sure that in your publishing contract you have the rights to your cover art. With your cover art you can make t-shirts, bags, posters and etc. You do not want your publishing company to steal the money because they own the cover art. Make sure that all works of your book belong to you, the author.
To get an idea of what your cover should look like look at books in your genre. If your book is a fantasy book, many of them have dragons or fairies on them, elements of the genre. Pull elements from your genre and in the story. Make sure that your cover does not copy another author's artwork.
By having an attractive cover you will attract more readers' attentions. Remember readers do judge a book by it's cover.