You will need to turn the layer into a 3D layer. Click on the box for the layer that is in the column with a 3D looking box. That will make your layer 3D.
Now just set keyframes for the type of rotation you want. If I understand you right, you will want to rotate around the y axis. Set a start keyframe, then move down the timeline a few seconds and rotate the object. It might be easiest if you just play with the settings to see how it works. The second keyframe with drop in automatically. There will be a point where the flat object is invisible since the object will not have any real depth.