Wipe off all of the wet surfaces after you finish your daily shower. This is the simplest method for shower cleaning. If you wipe out the shower stall and wipe off the shower head and fixtures every day, limescale and soap scum will not be able to accumulate. If multiple people use the shower, the last one out can clean it.
Clean the shower with a sponge and scouring powder every week when wiping it down daily is not feasible. If your water is hard, you might need to use a commercial rust and limescale remover in addition to scouring powder.
Heat up white distilled vinegar (do not boil it) and put it in a spray bottle. Spray it all over the shower head and fixtures, and let them soak for at least 15 minutes. Scrub them with a nylon-covered sponge and borax.
Rub the shower head and fixtures with a dry soap-filled steel wool pad. This is a method of last resort because the steel wool pad might scratch your fixtures. Do not use water with steel wool. This method should remove the buildup.