Hi @blodynlisa,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
To have a successful installation, you want to have a flat, clean surface to start with. In your situation, this will mean removing the adhesive that is currently on the wall. Depending on how strongly it is attached, you may be able to use a Floor and Wall Razor Scraper to remove it, but I think it is more likely you will need to use an angle grinder to grind it off.
Once the adhesive has been removed, you'll want to assess the surface to ensure it is nice and flat. If needs be, you may have to apply a skim coat of render to flatten the surface before moving ahead with your tiles. You want to ensure that the surface is nice and even, so every tile has full contact with the wall.
With your electrical outlets, there are two approaches. The first option, which will look cleaner, is to have an electrician come in and temporarily remove them, so you can tile underneath them. Once the tiling has been completed, they can return to reinstall them over the tiles. Doing this will mean the gaps in the tiles are completely concealed by the powerpoint itself, which looks much cleaner. The alternative to this is to tile up to them, which doesn't tend to look as clean as you would need to cut the tiles around them and then seal the gaps between the tile and powerpoint with silicone. It can be done, but it means that any time the cover plate needs to come off the power outlet, you would need to cut and replace the silicone.
Check out How To Tile a Splashback and How To Tile A Laundry Splashback for some guidance on the actual process of installing a tiled splashback.
If you have particular questions about this process, please let me know so I can assist further.
Jacob
