Start by removing the wallpaper thoroughly.
Do not even think about trying to tile over the wallpaper.
The short of it is simple: It won't work.
We had that and removed the wall paper by wetting and scraping it off. You do not want the wallpaper under the tiles. Once you have it removed then proceed as you would on any wallboard. How exciting to be making a new look in your home!
Hi Bedie, You may tile over existing tile, painted or unpainted drywall, plaster and textured walls. However, tiling over tile can add quite a bit of thickness, so make sure your wall can handle the weight. You should not tile over wallpaper, glossy surfaces, lead paint or plywood.