If you have an uninsulated door attached to your house insulate now. The easiest why to insulate you door is with Polyiseocyanurate Rigid Foam Panels. Available at Lowes or Home Depot
Panels do not add much weight to door so you can keep the same spring on the door.
Comes in 6’x 20” sheets
Double sided adhesive tape
Cut size to fit door.
Cost about $100 for a single door.
Insulate the ceiling especially if you have a room over the garage.