Treat your piano like a piece of valuable furniture: keep it clean and never stand drinks, vases of flowers, or pot plants on it. Spilled liquids can cause serious damage, the repair of which may amount to a major overhaul.
New pianos are generally finished in polyester or satin lacquer, which requires only to be wiped with a damp cloth and carefully dried. No polish is needed.
Older pianos are usually French polished and need greater care, using a polish reviver applied with a clean soft cloth and polished with a soft duster. Make sure to do a small patch test first. On no account use spray polish.