To find it, look closely around the power steering pump itself. The pump is usually located near the front of the engine, often near the lower portion of the engine. The reservoir will be mounted relatively close to the pump, and frequently connected to it via a hose.
It's usually labeled "Power Steering Fluid" or has a symbol representing power steering. If you have difficulty locating it, consult your owner's manual or a repair manual specific to your truck's year, make, and model. A picture in a repair manual can be extremely helpful. Online resources such as car parts websites might also have diagrams showing the location.