If you notice spots, mildew, drips or bulging drywall on your ceiling, you likely have water damage. It's important to repair this kind of damage as soon as possible to prevent a small problem or blemish in the ceiling from becoming a major issue, or to prevent something as serious as a collapsed ceiling. Many or all of the steps involved are things you can do yourself, especially if you have experience with drywall.
Identify the Source
Before you make any repairs, you must determine where the water is coming from. If you don't, the damage will simply happen again. With a ceiling, issues with the roof are the most likely source, though there are potentially problems with pipes in a multistory home, or with air-conditioning units. Leaks, structural damage, problems with the gutters, and problems with caulking and sealing are all potential sources of leaking water. If you're not sure of the cause, you can eliminate possibilities by determining if it leaks only when it rains -- if it does, it's a roof problem, and if not, it's a plumbing or air conditioning problem. If you are repairing damage from severe weather, the problem source may be obvious. Before inspection and repair, shut off any pipes or electricity in the area to remove hazards.
Remove Damaged Materials
After you have identified and, in the best case scenario, repaired, the source of the water, you should remove any areas of wet drywall or, in the case of a plaster ceiling, plaster. If the damage is light and you don't have any wet material, you should inspect the area anyway. This may mean getting into your attic. Look for soft wood in the structure -- an indication of more severe damage -- or mold or mildew, which must be removed for safety. Take care if you see bulging drywall; this can mean you have pooling water trapped in the ceiling, and this must be removed first. You can drain it from below by piercing through the drywall with an ice pick or a screwdriver and putting a bucket underneath.
Dry the Damaged Area
For a ceiling that is still very wet and heavily damaged, the next step is to dry the area. Put fans both above the ceiling and in the room below, if possible. Removing the damaged material and exposing the area behind it helps with air circulation and drying out the damage. Treat any mold that you see; there are many ways to approach this, but washing mold and mildew with diluted bleach is particularly simple and effective.
After dealing with the source of the damage, combating mold and getting the area dry again, the final step is to repair the damage. Installing new drywall or plaster is something you can do yourself -- but it may be too difficult for you to apply to a ceiling if you have no experience, because the ceiling is the most difficult area to drywall or plaster. For severe damage, you may need to replace insulation behind the drywall, as well, and replace or reinforce damaged wood. For minor damage, you can cut or sand the damage out and apply drywall compound to fill in the hole or crack. Then sand it smooth and paint. If your damage was not severe enough to warrant drywall removal and there was no mold, consider merely painting the ceiling with a stain-blocking primer and a coat of paint to cover up any water stains.