To install an adjustable keyboard tray you generally need a worksurface that is at least 1"-1.5" thick and can accept screws on the bottom side. This means most desks that are made of wood or faux wood materials are acceptable but desks made of things like glass will not work with most systems. Usually glass computer desks will come with some kind of built-in keyboard tray but if you happen to have one that doesn't you unfortunately don't have many options. If you have a normal desk and you don't mind drilling a few holes on the underside of it than you meet the initial criteria.
Keyboard trays mount to the underside of worksurfaces via a steel track. With most popular systems the mounting tracks are 5" to 6" wide and 17" to 22" long. You need an unobstructed flat area at least this size on the underside of your desk to be able to install most keyboard trays. If you don't have the required space there are specialized systems for this application but the options will be minimal.
Once you are sure your workstation meets the requirements for installing a new system you can then start to compare features of different brands. The general rule with keyboard systems is that the more you spend the better the product will be, and the longer it will last. It will also feel much better and be more comfortable during use. Some of the most reputable brands include WorkRite, Humanscale, Intellaspace, Kensington, Comfort Keyboard and 3M.