Growing up, our garage was Dad's domain. You were welcome to enter as long as you were just grabbing a skateboard or switching the laundry. Everything had a place -- car parts, tools, and enough nasty chemicals to eradicate a small nation -- and only Dad understood the organizational system.
So we don't know much about garages. We've never had one of our own. But, when we do, at least we'll know how to green it.
Since we aren't much help in this area, we recommend Grist's full-blown tutorial to help you start greening your garage. Of course, the best way to start would be to ditch the gas guzzler and park something like a Smart car in its place; however, Grist understands that most of us want to start with baby steps.
How about just taking care of some of the bad stuff in your garage first? Used oil, half-empty paint cans, etc? Grist will tell you how to responsibly rid yourself of the stuff.
Then you can move on to the bigger steps.