If the existing paint is peeling severely, you need to determine the cause before repainting.
Preparation involves washing the surface to remove loose dirt, chalking and stains.
Add some powdered automatic dish washing detergent to the bleach water to remove stubborn mold stains. Research from the USDA Forest Products Laboratory indicates that it is critical to carefully sand any bare wood. Just a few days of exposure of bare wood to sunlight changes the surface so that paint will not bond well. The longer it is exposed, the worse the problem is. It is, therefore, critical to sand off a thin layer of the exposed wood, about one millimeter, before painting wood that has been exposed.