Wood desks can easily be updated with a fresh coat of paint. Depending on the condition of the wood and existence of any previous paint, you may need to sand the desk in preparation for new paint; to make your job easier, an electric orbital sander works quickly and easily on large surfaces such as a desk. Use a coat of primer if you want a solid, clean coat of paint; if you're going for a distressed look, you can forgo the primer. Apply your paint with a paint brush or small paint roller, and after you let the first coat dry, sand it down for the smoothest finish possible.
Desks with a laminate surface can be painted with the proper preparation techniques and with the application of primer and specialty paint. Before you paint, sand down the laminate until you remove its shiny surface; be careful, though, to not sand too far, to the underlayer of pressboard or other wood. Apply a coat of primer formulated for laminate surfaces. This primer comes in paint-on and spray-on application types. After the primer dries, apply 1 or 2 coats of paint and a water-based polyurethane to protect the paint from scratches.
An old metal desk doesn't have to live out its life looking like it belongs in the corner of a factory; you can paint it any color your desire. Prepare the surface by roughing up the entire thing with an orbital sander. Apply a primer formulated for metal surfaces to the entire desk and let dry, then apply a paint formulated for metal surfaces. There are many spray paints made for this purpose, as well as traditional roll-on or brush-on paints.
Update a glass desk and hide smudges that seem to perpetually find their way onto it by painting the underside of it with a paint formulated for glass surfaces. This paint can be applied directly to the glass; prepare the surface by ensuring it's clean and dry--no other preparation is necessary. Because the paint is on the underside of the glass, it has an opaque look but won't get scratched when you place items on top of your desk.