Make the layer 3D, click on the little cube checkbox (switches column), the “R” brings up the rotation and you can now keyframe the X-rotation, by clicking the stopwatch, go down the timeline and put a new value in for x-rotation, you should have two keyframes now and AE will interpolate.
A good overview of AE basics is Andrew Kramer’s “After Effects 101”, here:
https://www.creativecow.net/articles/kramer_andrew/aftereffects101/ae_101.html