A book's cover—what a potential reader first sees—is the most important part of the entire design of a book because it has the power to make or break the sale.
The book that is most likely to be purchased has eye-catching appeal on the front that gets the customer to pick it up, and then enough information on the back to convince that customer there is something in between worth paying good money to explore further.
Other things that are part of the larger question of book design include interior, trim size, paper, photographs, illustrations, chapter title pages and subheads, font size, ink colour, footnotes, front and back matter, and typesetting. Unless you have personal experience in the design world, don't do this on your own.
It is good to have an idea of what you want your book to look like. Find a book you like that is currently on the shelves and show it to the person you have chosen to design your book. Let them, with their experience and design abilities, come up with a few concepts for you to choose from, make your choice, and then let the designer make it a reality. In the meantime, you focus on what you do well... writing!!