There are three types of belt tensioners used for serpentine belt systems. All of them involve a pulley. The first type is tightened or loosened via a regular hexagonal bolt on the face of the pulley. This is also the most common these days. Another tensioner pulley uses a square faced bolt for adjustment, and a third uses a large tab that can be set to a certain position for proper belt tightness. To remove an old serpentine belt, or to tighten up a new one, the tensioner bolt on the tensioner pulley is turned until proper tightness is acheived. Please see your repair manual for the recommended tension on your vehicle's engine.
You may also need to remove your radiator cooling fan to access the pulleys, They are usually bolted onto the back of the radiator or the radiator support, and should be fairly straightforward to remove.