Rob Mize demonstrates how to create virtual 3D sets in After Effects with some super simple techniques. Check out the tutorial for Creating Simple Virtual 3D Sets with After Effects here.
from the tutorial description: Rob Mize demonstrates techniques for creating a virtual set in 3D space. He shows how you can build sophisticated, detailed environments, where your actors appear to interact with their 3 dimensional surroundings. Build the set of your dreams using nothing more than After Effects, a few textures and your creativity.