What are the three best things you can do if you want to learn how to write?
Read, read, and read some more.
The three most important things you can do if you want to learn how to design a book cover are similar: look at lots and lots and lots of book covers.
But don’t just glance at them like you have until now. Analyze them. Take them apart. Figure out what makes them work–or not work– and why.
But you’re a writer, not a cover designer, so why do you need to do this?