Figure 8.1 - Track To Constratint
At the options, you must specify a target object that your camera must
follow, and also the axis used for tracking:
To: Use the -Z option to make your camera look to the object.
Up: Here, you have to use the Y option to align the camera Y-axis
with the world Y-axis. Don't change this option unless you want to
rotate the camera.
Now you have the camera following all movements made by the target
object. If you use the Object → Track → Track To Constraint menu,
the target object will be set automatically. For that, you must select the
target first and the camera last.