If you have a greenhouse you’ll want to let plenty of winter light in so cleaning the glass with warm water and a little washing-up liquid will clear grime and any green slime. Clean the rest including all metal and woodwork by adding disinfectant such as Jeye’s Fluid to the water.
Change the soil in a greenhouse border annually if possible and replace with good loam or the best garden soil. Check all electrical apparatus in the heated greenhouse. Use bubble wrap to line the interior and conserve heat.