Hi,
In order to animate the head you can either use Physique or skinning. To physique, obviously create a biped and align the biped head to your head model. Then select your head model and go to the modifier panel and select physique. You should see the following, Floating Bones and physique. Click the little plus sign to reveal 5 icons. 4 of them are greyed out. The one you want to click on is the one that isn’t greyed out. So click it. Go to the select by name arrow located at the top toolbar and select Bip01. A physique intialization box will pop up. Just click intialize. Now select the biped head and use the select and rotate tool located at the top toolbar and move the head. The head model should move with the biped. You may have to play around with the envelopes if part of the head hasn’t been physiqued correctly to the biped head.
To skin, you select your head model and in the modifier panel you select skin. You should see the following, Parameters. If it isn’t opened already click on the plus sign to extend the parameters roll out. You will see the edit envelopes button and scroll down until you see the bones and then the add, and remove buttons. Click on add and select the Bip01 Head. Now select the Biped head and using the select and rotate tool move the head. The head model should move with the biped head. As for the envelopes part of skinning I’m in the process of learning that. I hope this has given you some insights. And welcome to the boards.
EDIT: One more thing, you may have better luck using the Skin modifier rather than physique. Physique hasn’t been updated in like 300 years lol:D