Now before there is a kerfuffle in the comment section, I am only suggesting that you do this with the cheapest and most hopeless guitar you can find for three bucks at the thrift store. Once you have one, do what Recyclart member Neokentin did and take the back off. This can probably be achieved with a razor knife, a sharp chisel, or a rotary tool with a cut off disc attached. It is just glue holding it together so once you break through the seam in one area you should be able to carefully pry off the rest of the back.
Once you have the inside exposed, it is up to you to decide how many shelves you will add and where they will go. Glue small scraps of wood for shelf supports in the appropriate places and screw the whole thing to the wall. But before you get too far ahead of yourself, you have to know at what angle the guitar will be placed on the wall. This information will determine the proper placement of the supports to ensure level shelves. Good luck and rock on!
See more: Recylart