Insulating a garage may sound unnecessary, but if it's attached to your home, you could be losing a lot of valuable heat between those bare studs and rafters. You should be able to seal up a standard garage in one day with proper planning and a good electric staple gun. Use fibreglass batt insulation (those big, thick rolls) with a paper vapour barrier backing, particularly if you think you might not immediately bother with drywalling.
Put on your long sleeves, gloves, dust mask and goggles. Starting in one corner of the garage, unroll enough insulation to hold it up to the ceiling between the first two studs on the wall. Press it in with the bare side facing the wall and the paper backing facing out into the room.
Use your staple gun to attach the paper strips (on the side of the insulation) to the sides of the studs, just below the outer corner of the wood. Put one staple about every foot, all the way down both edges of the strip. The surface of the paper should be roughly even with the outer width surface of the stud.
Use your razor knife to cut off the strip of insulation at the bottom of the wall. Move to the space between the next two studs and repeat the process. Continue around all the walls in the garage, using your razor knife to cut the insulation around outlets and other obstructions.
Move to the upper part of the garage, standing on the cross beams so you can access the rafters running up the inside of the roof. Hang insulation between the rafters in the same fashion as you hung it between the studs below.
Don't handle the insulation without gloves and long sleeves, as it is a skin irritant. Keep your dust mask and goggles on while working with it, and keep the area well-ventilated. Use caution with cutting the insulation to go around electrical fixtures.