So, your living room looks like a cave. Your 16-year old thought painting the room a deep shade of grey would make him more creative. Or, the new house you moved into is lovely, except for the master bedroom, where the paint makes it look like a starless night. The good news is that covering dark wall paint is moderately easy, although it will require some work and a bit of time. But in no time, that dank room will be bright and look like new.
Wash the walls. Before any paint is applied, it's important to give the walls a good scrub with dish soap and water in order to remove dirt, grime, or grease that has accumulated. Use a sponge to clean the wall without damaging it.
Prepare the room. Sand rough spots on the walls -- sandpaper with 120 grit should do -- and patch any holes by using a putty compound and a putty knife. Put down a layer of plastic sheeting to protect your floors, and cover any furniture or items that are still in the room. Finally, put painter's tape over any trim that you are not painting, and along the ceiling, unless you're painting that, too.
Apply a primer paint to the walls. For an interior paint job, on most surfaces, an all-purpose acrylic primer will likely be your best choice. This step is essential, since it will cover the dark paint, and decrease the chance that dark spots will show through when the new paint is applied. Use a roller to evenly cover your walls in primer.
Choose your colour and paint type. If you're trying to brighten the room, you will need a brighter colour of paint. You will also want to use a latex paint, rather than an oil-based paint, since it's easier to apply and to clean up.
Once the primer has dried, begin to paint your walls using your chosen colour. You can put as many coats on as you would like, just make sure to let the previous coat dry before starting another. It may take two, or even three, coats before the room is as bright as you would like it to be.
Clean up. Remove the painter's tape and tarping, wash your brushes, and replace anything you have taken out of the room. Those dark walls should now be a thing of the past!
Try to make sure the room has good aeration that allows the paint to dry evenly and more quickly. A fan can be helpful, but should not point directly at one spot within the room. It should be positioned to keep air moving throughout the room.