If you live in a hard water area and have water marks on your stainless steel sink, pour some clear neat vinegar (or lemon juice) on to a paper towel, apply to the stains and leave a few hours.
If you’re feeling energetic, mix a paste of bicarb of soda and vinegar and smear on with a damp cloth or sponge.
Leave a few minutes, rinse off and buff dry.
Once clean, get into the habit of descaling the sink on a weekly basis, paying particular attention to the bits around the taps.