Most surfaces allow you to paint over them. The application process for each of those surfaces is different though. In short, you need to choose the proper paint for the surface, based on what type of surface it is. If that sounds confusing it is because the process can be. The good news is that you can easily get great looking surfaces into the colors you want by first investing in the right type of paint.
Painting wood and other porous surfaces is not as difficult as other types. The benefit here is that these surfaces will easily soak in the paint you are applying on them, even if they have paint already on them. In some cases, it is best to use a chemical solvent to dissolve previous layers of paint first before applying paint over the top. Most finishes and paint types will work fine over the top of wood, drywall or similar porous surfaces.
A bit more challenging is metal. Some types of metal will easily allow other paints to pick up on them. If you need to paint a metal fence or handrail, for example, you can usually doo this fairly easily. For the best application success, be sure that all rust is removed. Run a light sand paper over the top of the metal and then use a spray paint rather than a brush paint. You may need to place a primer coat down especially if the paint is lighter than what is already on the surface.
Painting tile is a bit more difficult. Tile is usually highly glossy which means that paint, as well as other materials, just slide right off of it. This is fine in most cases since it helps you to keep the tile clean. But, if you do want to paint over tile, you will need to find a tile paint. These are sold in most paint specialty shops and online. These paints are able to grab onto the tile better so that the finished product looks great.
If you are trying to paint something that is glossy, it is best to use a layer of flat paint first and then apply the paint you are looking for. That flat paint will form enough of a grip to hold the other layers of paint in place.
Most surfaces do allow you to paint over them, but you do have to be careful with the types of paint you use and the methods of application you use. A professional look is possible in most situations if you take your time and do not over apply too much paint on the project. This is a fantastic way to update the look of any space inexpensively, too.