Remove the wall paper and prime the walls flat white. If tile is ceramic than you can use a premixed mastic such as "TEC Double duty". If you are doing glass tile than you need to use a very high quality glass tile mortar such as "TEC Superflex". Your job is as only good as what you stick it to. The wallpaper will loosen over time or as you are installing the tile (not good). You will need longer screws for the outlets.
