Were working backwards from installing tiles. First get a wood chisel and carefully remove the tiles in a prying mode. You only want to be careful not to save the tile, but to prevent dents and holes in the wall board.
Removing the tiles are very easy, once you remove one just start prying down.
Next, you have the mastic that either stuck to the tile backs or to the wall. Depends on how much was applied at the time of installation.
Get a stout putty scraper and try to remove all the mastic from the walls as possible. A sharp wood chisel will really help.
PS, these wood chisels should be old ones that you have sharpened for this occasion or new ones that you do not intend to use again on its designed purpose.
When you have removed all the mastic from the walls that you can, then begin the sanding process. Use a coarse paper and sand away.
Then after sanding, access the smoothness of the walls and blemishes from these steps.
It's your call. I would refloat the wall area with new mud and let it dry and then sand. Then do it again then add your texturing to match if necessary the above wall section.
Then apply a mud or texturing primer for paint adherance. Use several coats.
Then apply your finish coat and new base boards and you are ready. Sherwin W. makes several good lines of paint for bathrooms and kitchens where you will and might have condensation or steam.