How do you wash walls before painting?

Repair and spackle any holes in the wall. Remove switch plates and mask the outlets or switches. Cover the floors. Clean the walls if they have a build-up of grease, dirt, etc. using TSP or other cleaner that does not need to be rinsed. If there is smoke damage or any other type of staining or odor, apply a coat of Kilz and let it dry, then apply the paint. The Kilz also prevents color bleed-through if the previous paint is darker than the new paint color.