Without seeing your bathroom it's a little hard to tell you how to get up there but I've used a little laundry detergent and a fair amount of bleach (about 3/4 cup per gallon of water). Scrub the ceiling well, leaving the surface damp to allow the chlorine to work on the mold. It may take several applications. You will want to wear old clothing because the bleach will ruin them and remove anything in the bathroom like towels, rugs, and shower curtains that could be damaged by the bleach. Eye protection isn't a bad idea either. There are also special mold removing chemical available at Home Depot if you want to go that route too.
The most common cause of mold in a bathroom is improper ventilation. Having a properly sized exhaust fan and putting in on a timer so it runs 15-20 minutes after bathing will really help to eliminate moisture and mold. Make sure the fan ventilates outside your home, not into the attic.
If you are going to repaint the ceiling I would highly recommend Zinsser's "Perma-White" paint. It is very mold resistant (but it alone should not replace good ventilation). Of course, let the ceiling fully dry before painting.
Thank you very much. It is not on the ceiling but it is in the crease of the ceiling and wall. I should have taken a picture and posted it. I use the exhaust fan but I found from reading your comment I neede to leave it on longer and that is what I will do next time.
The best and safest thing The Home Depot sells for cleaning, killing, and preventing mold from coming back is Concrobium...sold at your local Home Depot's paint department.
Shown below and linked here, all you need to do is spray and wipe off after a few minutes to ensure the mold is completely removed from where you applied this awesome product. This product is so effective and safe to use, that you can indeed spray your old clothes with it to eliminate any musty odors/mold on them.
Bleach, even diluted, may not kill all of the spores as effectively as Concrobium can and will. Try out a bottle of this, and then you can paint/prime your walls. Be sure both primer and paint are mildew-resistant, and you will no longer have mold issues!
Let us know if you have any further questions, Joseph
Thanks so much John. I will definitely come and pick up mold control and see if it works. Is it best to paint the whole bathroom or just the mold area? Can you answer this question? How can I reach the area which is at the seam between the ceiling and wall. I cannot be a ladder in the tub. It is not stable enough. Would a mop do the trick?
Interesting that there is no way to reach your tall ceiling.
I will assume that you will need something like a "multi-position" type ladder. Think some version of a "Little Giant" or one of their competitors. Multi-Position Ladders
In addition, you might be surprised at how the Concrobium product works.
It is a mixture of liquid salts that kills mold by encapsulates and physically crushed the mold as it dries.
If you can find a way to simply spray this on the moldy areas above your tub, then all you really need to do is turn on your fan or open available windows and let the product dry. Only then would you need to consider using a long handled mop to clean off the surface. You may not need to. See this link on Concrobium.
As far as painting is concerned, spot or area painting rarely looks right when done. Try the mold killer and let it dry.
Then we can go from there to figure out how the tall ceilings were painted in the first place.