There are various cleaners on the market that are specifically designed from the removal of Mildew. Many of these work quite well, however, a bleach and water solution will also work.
Wash mildewed areas with a solution of one part household bleach and three parts water. This will destroy mildew and bleach stains caused by mildew growth. Apply solution by brush or a garden sprayer apparatus. Heavy mildew may require additional applications, and scrubbing may be required. Flush area with clean water (before the solution dries) to remove bleach solution. Allow the surface to dry thoroughly. Be sure to use protective clothing and goggles to avoid contact with the solution. Do not mix bleach with other chemicals.
Please note that these steps address the mildew problem only. Any other contaminants may require cleaning by other means.