One of the first things you will discover when self-publishing is that you can't do much without a cover. A book without a cover is just a stack of pages after all. A cover allows those pages to be encased in a package that is easily shelved or flipped through. It allows someone to see the title from a distance, and get a quick gist of the contents by a simple look.
Now, the options for cover art range from the mundane to the most lavish and eye-catching. The simplest involves the title (which may or may not have colorful and exotic font), author name, and any other identifiable information. For the more inclined, one can even throw in a few images from the public domain (those that are royalty and copyright free). CREATOR BEWARD: the term "public domain" is relative because there could be caveats to that picture: it can only be allowed for personal use and not widely distributed. Or it could only be used a certain number of times. That is why it's always important to check the fine print.
Provided you are able to locate a public domain image that is free of any gotchas, then it's a "simple" matter of using a problem like Word or Microsoft Paint and add text over, and then boom: you have yourselves a cover!
Of course, having something simple is all right if you aren't interested in marketing it. If you just want to get it up there, then a public domain image on your cover will do. But if you love your book and want the cover to properly represent an ideal found within its pages, or want people to look at the cover and say, "I want to read this!", then you're going to have to fork over some hard-earned capital or hard-earned time.
I say either because if you have some inclination to use photoshop, then you could purchase some high-def images from places like Shutterstock or DepositPhotos and form them into a proper cover. Of course, you should always still be aware of the fine print with these images so you don't violate licenses and copyrights.
And if you don't have the time or experience with photoshop or similar software, then you can go the purchase route and have someone create cover art for you. Assuming you don't have a wonderful friend who'll do it for a few pints, then you have two options: 1) premade art; 2) commissioned art.
Option 1 involves art that someone has created and posted onto a website like TheBookCoverDesigner.com and RockingBookCovers.com. The cover can be purchased for a reasonable fee and then the artist will put your title, name, and tagline or subtitle on it. For an additional fee they'll create a full cover (in the case of physicial print) or do limited modifications (like add a flower here or a spiderweb there).
BEAR IN MIND: this type of art is not fully customizable! You are paying for something already created and do not get to tell the artist how to change it. I sadly made that mistake with one of the artists and they got upset (rightfully so!). Some artists will insist you pay additional costs to make any further changes, but if they do not, then you are stuck with what you have. And if you don't like it, then you should stick with Option 2.
ANOTHER IMPORTANT NOTE: premade art is a one-time deal. If you purchase it, it's yours because by policy, it cannot be resold ever again. A lot of the above sites won't even accept a previously sold piece. Moreover, the sites that you purchase it from take a huge chunk out of the sale, so if you can contact the seller directly, DO IT! They'll thank you in the end, and they might even throw in a free customization. That's a lesson I learned the hard way.
Option 2 involves fully customizable work. That means you start with an idea and work with the artist to manifest it into life. Usually these artists will use something like the aforementioned high-def image sites as a basis and then manipulate and layer the images until the result is a finished book cover. The more artistic-inclined people might even draw something. Either way, you are paying for a custom book cover so it will be at a premium.
The cost of a custom cover can range greatly, from a little over a hundred to almost a thousand dollars. It all depends on what you are asking from the artist and how much time they will need to spend to complete your request. Remember, someone has to spend time to create the art, whether it's you or them. If it's them, then they will charge you for it.
With custom work, you do have a little more leeway into what you can ask for, but again don't go crazy or you're liable to find the final price tag is completely outside your budget.
So depending on what your budget is, you can go with simple art, self-made art, pre-made art or custom art. Simple art won't do much in the marketing department, but it's cheap and easy; self-made art will allow a little bit of pizzazz, but it's time-consuming; pre-made art can be great if you find the right image, which is tough; and custom will get you exactly what you want but at much higher cost.
Choose whichever you feel is right for your book but bear in mind that whoever does the art (be it you or someone else), they are spending their hard-earned time to create a finished product. So respect them and the art they provide, because after all, artists are people too.