the most straight forward way to animate something like this is to parent the 3d layer to a 3d null, offsetting the 3d layer from the nulls position and then rotating the null around the relative axis to give the rotation needed.
to keep the layer facing the camera, select the layer and choose layer>transform>auto orient, and check the option for ‘towards camera’
if for some reason you are not able to use this technique, expressions can also be used, but the null method usually works and is easier to manipulate.
— edit —
i should’ve mentioned that orient towards camera also needs a camera in the comp
Kevin Camp
Art Director
KCPQ, KZJO & KRCW