Remove all the items in and around the shower, so mold spores do not infect them.
Fill a spray bottle with one part distilled vinegar to one part water.
Put on a breathing mask and eye protection. Spray the ceiling with the vinegar solution. Scrub the ceiling with a brush to remove all the fungus. Dry the ceiling with old towels.
Analyze the cause of fungus on the ceiling. Most of the time, a bathroom ventilation system that is working correctly and used regularly will alleviate the problem.