This can depend on the material of your seat. Different material has different requirements so that it can remain in the best possible condition.
Wood needs tender love and care when cleaned, so using a mixture of warm soapy water and a scrubbing brush, you can scrub away any dirt with circular motions. A little bit of bleach and warm water can be used for stubborn areas – just rinse off with fresh water.
Oxidation and rust are common with metal furniture, so a solution of one part vinegar and one part water can be used to prep the material. Just scrub it down with warm soapy water and a soft cloth. Make sure to dry it afterwards.
Plastic garden seats can go green with time – restore it to its natural colour with a mixture of one parts vinegar to two parts warm water, gently scrub it, and hose down. Bicarbonate of soda can be applied to stubborn areas. Dry afterwards.