Bill, we assume you are referring to a sectional garage door rather than the tilt-up type. Generally speaking a good quality sectional garage door that has been properly installed is reasonably weather-tight. But even the best doors can begin to leak after long use. A simple fix would be to apply a one-inch wide strip of rubber to the inside surface of the door at each horizontal section joint -- all the way from one side of the door to the other. Apply the strip to the upper section centered between it and the section below. A special rubber bottom can be purchased to fit the base of just about any garage door. For the sides of the opening -- on the outside trim ? attach good old-fashioned weather stripping for a conventional door. You know the kind don't you? The type with the round vinyl bead attached to a flat metal strip. If the door is wood used staples or nails to attach your wind proofing. If the door is metal then use half-inch, self-tapping screws. Once you have all of the materials you should be able to do the entire job in an afternoon. And, good luck!