Firstly make sure you have a set of teeth that fit into the mouth area, making sure the pivot point for the lower set of teeth is rotating from around the lower ear area where our own jaw tends to rotate from.
You have quite a few choices on how to make things work, but I'll try and describe how I prefer to do it and that is to create a joint chain for the head with nicely painted weights for neck twists/bends and so on. I then just parent the teeth to the rig. I then create my mouth opening with blendshapes in various forms such as open, shout and so on. I prefer to then connect these up via set driven key menus with the blendshapes driving the jaw rotation as it opens and closes.
Another way is to make a joint chain again as mentioned above and also bind the teeth to them, then you will have a straight forward and very simple way to open and close the mouth, you can then again add either again blendshapes or even add more joint or clusters around the different parts of the face to animate the different areas.
hope this gives you a better insight, there is a set driven key tute on here that you may want to check out