If you want to tile over an old plaster wall, and it's not a bathroom or other wet room, you can make it work, but only if the wall is smooth and even. Old plaster walls tend to be uneven, and those ridges, bumps and holes will compromise the bond between the tiles and the wall. Patching the wall and smoothing out the ridges will give you a flat surface to tile.
Push your drywall knife over the surface, with the long edge pushing forward, to remove any bumps or high spots.
Spread plaster patching compound with your drywall knife over any parts of the wall that have cracks, holes or depressions. Scrape the long edge of the knife over them to get a straight, flat surface.
Spread additional plaster around the edges of any ridges that are too big to be scraped off. Flatten out the plaster from the top of the ridge outward, to reduce the angle of the ridge.
Let the plaster dry for a day. Sand it by hand with a drywall sander and fine paper. Wipe off the dust.
Mark a horizontal line along the bottom of the wall with your level and pencil, making the line the height of a tile plus 1/4 inch. Mark an intersecting vertical line at the centre of the wall and extend the line from the bottom to the top of the wall.
Apply thinset mortar along the bottom, making it slightly higher than the horizontal line. Set the bottom row of tiles in place under the horizontal line, starting at the vertical line and building out toward the sides. Put tile spacers between the tiles. Cut the end pieces as necessary with the tile cutter.
Build up the wall row by row by starting each row at the centre vertical line and building out toward the ends, until the whole wall is tiled. Let it set overnight.
Remove the spacers. Grout the wall from the top down, spreading the grout with a grout trowel and pressing it into the lines between the tiles. Wipe it down with a damp sponge to take up the residual grout.
Wear protective goggles when cutting the tiles.