First of all, are you talking about walls, shower, tub surround, or all of the above?
If you're going to replace old tile with new tile, plan on taking out the drywall with it, and replacing it with cement board (the best surface for tiling).
If you are doing a shower, or tub surround with shower, keep in mind tile and grout are not waterproof. You need something to stop the water from penetrating into the wall. There is more than one way to do this.