How to prepare a wall for tilling?

I've been watching this question gather dust for a couple of days.
I think that nobody's touched it because nobody wants to give you the bad news.

You can't prep an existing wall to accept tile. You can only stick tile to new work.
If there's any sort of paint, plaster, adhesive from old tile or wall paper or an old "glue up" type surround,
the tile isn't going to stick.

Once you've picked out the tile, you need to find out what the manufacturer's recommended adhesive for that tile is.
Next you need to check with the adhesive manufacturer and see what the recommended substrate (backer board) is.
This could be anything from a special type or wallboard coated with treated paper or a fiberglass mat to a sheet of cement sandwiched between 2 layers of screen. Some tile even requires a wire mesh and a skim coat of cement.
Then you're going to need to take all the existing wallboard down from where ever you plan on installing tile and put up the recommended backer board
THEN you can install the tile.