Generally the best first step for a DIY project is to start sealing up the holes in your house.
Check all your exterior penetrations and use a can of spray foam to seal up any holes.
Then check your garage-to-house attic connection.
- If you can clearly see from your garage attic to your house attic, then you need to run a sheet of drywall or plywood on that wall all the way to the roof deck and insulate that wall on the house-attic side, and then drywall that cavity.
- Remember to caulk the top and bottom of the drywall to complete the seal.
Filling and sealing all of your existing holes should be the first thing when trying to green up your house.
For more information:
Read "Should we insist that our contractor use expanding foam insulation around our new windows?" a Q&A answered by Carl Seville.