Step One: Watch Mary Rose Cook Live Codes Space Invaders from Front-Trends. (The second worst conference name ever?)
Step Two: Fork this gist.
Step Three: Respond to this question in your fork: What is one approach you can take from this Mary's code and implement in your project?
Collision detection. If we just figure out left, right, up, down and diagonals. Then we can go through and figure out if it is to the right or left of something (based on the edges of two bodies).
Step Four: Totally Optional: take a look at some of the other forks and comment if the spirit moves you.