It sounds like you are probably getting nailed in a few ways.
The biggest is probably your attic which may not be insulated above the garage. Even if it is very few will have a cover in place.
The second one is of course your garage door. You can buy a light colored insulated one. Or if you are happy with what you've got, go to your local building store and buy some of the pink or blue insulated sheets they have there. Cut them to fit and apply some adhesive. Both of those methods will help slow down the amount of heat that you gain during the summer and lose during the winter.
Do not consider placing an HVAC supply in that room as you would be introducing all sorts of bad things into your house including carbon monoxide.
Beyond that all I can do is recommend you get an energy audit, especially if you have a water heater or HVAC unit in that area.
For more information:
Read "Is a garage door insulation kit worth the time and money? Will the insulation make a difference in temperature for a room above it?" a Q&A answered by Lee Hall.
Also, read "Air Sealing: The Attic Hatch" on Sean Lintow's website.