Mix a three to one solution of warm water and white vinegar in a spray bottle. White vinegar is a powerful natural cleaning agent that is very effective for mold removal.
Spray the sheet rock ceiling with the water and vinegar mixture. You don't want to get the sheet rock too wet, so the spray bottle is useful in covering the mold with an easy-to-remove mist.
Scrub at the mold stains with a scrubbing sponge or brush. If you have trouble with stubborn mold that isn't coming off, try an undiluted application of white vinegar and let it sit for a minute before scrubbing.
Dry the sheet rock with a rag. The friction from the rag can also remove any remaining mildew.