The Size of the Job
The U.S. Environmental Protection Agency recognizes three sizes of mold infestations, and recommends that amateurs attempt cleanup on only the two smallest. A contamination area of up to 10 square feet is a small one, and provided the mold is all on the surface and accessible, you can usually clean it up with soap and water. A 10-to-100-square-foot contamination area is a medium one that requires a certain amount of containment, as well as expert experience or guidance. Anything larger than 100 square feet should be handled by a professional. It requires large-scale containment and remediation requiring specialized training and equipment. This type of infestation typically follows a disaster, such as a flood, storm or major leak.
Surface Mold and Mildew on Drywall
Mold and mildew -- which are basically the same things -- can grow on the finished surface of painted or wallpapered drywall when the humidity in the room is high; you may find it in a corner of a poorly ventilated room or in the basement. The most important thing to do is to reduce the humidity in the room by providing ventilation, dehumidification or heating, or your cleaning efforts will be for naught. Wear a respirator and gloves while working, and wash the mold with detergent and water -- you don't need bleach. Spray powdery mildew with water before washing it to prevent releasing spores into the room when you scrub with a sponge or cloth.
Dealing with Damaged Drywall
Blackened drywall that is spongey, warped, wet or otherwise damaged must be discarded -- there is no way to kill all the spores that have infiltrated it. Cut out an area around all such damaged drywall with a drywall saw, ensuring the area allows you to completely remove the damage, and put all the drywall in heavy plastic bags. To prevent contaminating the rest of the house, seal off the area in which you're working by hanging plastic in the doors and windows and over the air vents. You'll probably find damp or blackened insulation behind the drywall -- throw that away as well.
Before you re-insulate and hang new drywall, thoroughly clean the framing with soap and water and give it plenty of time to dry. You must also address the source of the moisture, and some sources are harder to control than others. For example, you may have a concrete foundation through which moisture seeps from the ground. If you suspect that your moisture problem may recur, consider hanging mold-resistant drywall, a typically paperless and slightly more expensive drywall product. This is not a substitute for preventing moisture in the first place, but it can limit mold growth if moisture becomes present.