Inside, insulate the walls and the door.
Purchase insulation from a big box home improvement store. They can help you figure out how much you need. Apply weather stripping around the garage doors and walk-in door. It will help insulate a lot, too.
Weatherproof? Is water getting into the interior of the garage?
Are the doors and windows fitting properly?
Do you mean moreover how to insulate a garage?
Not sure I understand. Are the elements getting inside or do you just want to insulate it?