What is the existing paint? How long has it been on the current surface? Is this on a wall, or something else? Is there texture? Is this inside or outside? What is it that you… want to paint it with? .
You are going to have to supply us with much better information if you want a good answer. .
Normally, you need to try to figure out the answer to question 1 to see what is compatible with the existing paint. The problem that you will have will be adhesion of the new paint. With some surfaces, you can use a special chemical that will make the surface temporarily "open".....so that it allows the new paint to adhere better. These products have a very strong odor though, and you would not easily be able to use them indoors. Another method is to sand the surface to "rough it up". You would go over every square inch of the surface to be painted with a light sandpaper or emery cloth. Frankly, that is the BEST method...but if you are trying to paint walls that are textured, you will tend to only sand the high spots off of he texture. .
If the existing paint is a high gloss latex paint, and it has been on there for a while you will do fine to clean it with a product like TSP (Tri Sodium Phosphate), then paint right over it with another water based product....or better yet, clean it, then use a barrier coat like solvent based Kilz, then your finish coat(s). .
If you are looking at interior walls....ask Sherwin Williams...seriously. Go to the paint store (Other paint stores will do as well), and tell them what you want to do, and what you have. They will have product recommendations that should work well. The guys at the big box stores want to be helpful, but the people at the paint stores are much more dedicated and knowledgeable. ( Full Answer )