I had an old computer case which would not hold the new ATX board. Rather than throw it away, I put it up on the wall so that at least some of my junk can be kept off the floor.
The first step is to hammer a nail into the wall. I was not allowed to, but luckily there were two nails already in that wall. So I decided to do a hanging shelf on a single nail. To hold it steady sticky tape was fixed to the wall at strategic locations. This helps the shelf stay in place even when loaded unevenly.