If you're planning on tiling an old plaster wall, it might be a good idea to first install a layer of cement fiber board over the plaster. If the wall is in a bathroom, it's a virtual necessity. Cement fiber board looks like a harder version of drywall, but in fact it's much different. Rock-solid and waterproof, it's the perfect material for tile backing because it won't move and won't soften if moisture gets to it. The boards have to be secured directly to the studs, not the plaster, so invest in an electronic stud finder.
Locate the studs in the walls, using your electronic stud-finder. Mark out the studs with your pencil and level, from top to bottom throughout the wall.
Measure and cut your first sheet of cement fiber board so the vertical edges of it will run along studs, as it sits lengthwise along the floor. Cut it with your jigsaw, making the cut on the side facing a corner, rather than in the middle of the wall.
Spread carpenter's glue on the back of the cement board. Set it lengthwise along the bottom of the wall, lining up the cut edge with the side wall and the factory edge with a stud. Secure it by sinking screws every 6 or 8 inches along each stud behind the board.
Repeat the process for each of the other boards, working your way across and up the wall and making sure there are no four-way intersections between the boards. All seams between the boards should be factory edges, with your own cuts facing the side walls. Make any necessary cuts for plumbing fixtures or other obstructions, using your jigsaw.
Lay mesh drywall tape over the seams between the boards. Spread thin-set mortar over the drywall tape, getting it flat and smooth. Let the thin-set mortar dry for a day. The wall is now ready for tiling.