So at the first sign of the sun we turn our attention to getting back out into the garden, morning coffee, afternoon tea, a sundowner! all very enticing but your garden furniture is in a bit of a state after the winter. You can soon have it 'well earned cuppa' ready with a bowl of warm soapy water and a bit of elbow grease.
Soap and water should be your first step, this will work on wooden furniture, with or without paint, metal, rattan and plastic.
Make up a solution with your soap of choice and warm water.
Apply to the surface and scrub gently with a soft brush or a cloth.
Rinse with clean water and let dry.
Its more often than not as simple as that, if however its proving to be a bit stubborn to shift you can try sugar soap, its very good at removing grime, grease and flaky paint, be very careful as you could easily remove too much paint, always rinse thoroughly.
Metal furniture might have some surface rust on it, this is usually easy to remedy with some wire wool and some household cleaning items, try baking soda, lemon or vinegar in place of the soap and water and rinse thoroughly. You can buy chemical rust removers for more stubborn rust, you would have to follow the particular instructions for the brand you buy.