If you are planning to update your bathroom with some of the latest porcelain tile designs then you will first need to remove the old ceramic tiles. Depending on the substrate they were originally installed on and how well (or badly) they were installed this will affect how easy or difficult they are to remove.
Very old tiles that were installed using a basic adhesive may have started to come loose and can simply be removed by inserting a putty knife behind them. But if the tiles are not loose then some of the grout will need to be chipped out first with a hammer and chisel and then the same tools used to remove the tiles. Alternatively you can use a claw hammer to force the tiles off the wall. Remember to wear safety goggles to protect your eyes from any stray chips of grout or tile.
Sometimes ceramic tiles have been cemented in place and then a claw hammer or hammer and chisel may not be adequate to remove them and some more brute force will be required such as from a pneumatic hammer. Whatever tools you use to remove ceramic tiles, if they were well stuck to the walls or floor, then you risk causing damage to the substrate while removing them, particularly if it is plaster.
There are also likely to be remnants of adhesive, grout or tile left on the undamaged areas that will need to be scraped away. If the damage is minimal then you might be able to simply sand the substrate before re-tiling, using a levelling compound or thick-bed adhesive is there are any uneven levels in the surface. But if the surface is badly damaged and there are deep uneven sections or large holes in the substrate, or if there is loose plaster, then it will need to be filled and sanded before the new tiles can be installed. For very badly damaged walls it may be necessary to completely replaster the wall but it is important to prepare the surface properly to ensure you achieve perfectly tiles walls.
Sometimes a better solution to removing very solidly fixed ceramic tiles is to tile over the old tiles. This will obviously make the walls thicker and may impede the installation of new fittings particularly in bathrooms which tend to be small anyway but this might be worth considering if removing the old ceramic tiles is proving difficult.