For mold on walls, check to see where the moisture is coming from. Some sources include leaky pipes, roof compromise and increased interior humidity. Add more insulation to prevent condensation that may be a source of mold growth.
To clean existing mold, gather the needed supplies. Buckets, water, bleach, sponges, scrub brushes, plastic sheeting, tape, towels, old clothes, gloves and safety goggles may all be needed. If the drywall needs repairing or repainting, gather those supplies as well. Bleach is a good choice for cleaning and removing mold, but commercial mold-removing products are also effective.
Use plastic sheets or drop cloths to protect the floors and other portions of the walls from the bleach. Tape them in place. Turn on fans and open any windows to provide adequate ventilation. Put on old clothes, gloves and safety goggles. Consider a mask if bleach is irritating.
Mix a solution of three parts water and one part bleach in a bucket. Use the sponge to apply the solution to the moldy walls. Scrub hard to remove the mold. For tougher mold and mold stains, a scrub brush may be needed. Let walls dry, and check for new mold in a few days.