ideas for insulating garage/wall ceiling covering

Do you have vented soffits? If you don't, you will need to vent them before you insulate, and will need to add gable vents if you don't already have some. Add baffles on the soffit ends to keep the insulation you add in the ceiling from stopping up the air flow. You need air flow above the insulation to keep your roofing shingles from baking off from the heat below in the attic after you close it all up. Insulate it with paper faced fiberglass batts, with the paper facing into your work space. I would do sheetrock on the inside, but that's just me.