The background comes in many ways, it's the main setting in the graphic. A background can be anything like a castle, forest, jungle, clouds, street, alleyway or anything else. I usually download stock photos like this pack from DeviantArt.
They are many helpful packs like these, you should really download them. You can even simply add a model if you are a beginner and don't want to get in a mess for the first time. Like the following....
ESTÁ A LER
Photoshop tips and tricks
Outros génerosPhotoshop explained in a easy way. Tips, tricks, tutorials and videos in this book. By the time you finish this you will be able to make your own book cover!