Check the rain gutters on your home. Put on a pair of work gloves, and climb a ladder to examine the rain gutters around the garage. Clean out any leaves or other debris clogging the gutters. When you are finished, rinse the gutters out with a hose. Rainwater will spill over clogged gutters, pool around your house and seep into the garage.
Plunge and clean out the downspouts of the gutters with a broom handle. Clogged downspouts will cause water to back up in the gutters, and the overflow will often accumulate around the base of the garage.
Measure the length of the bottom of the garage door. Cut a piece of weather stripping to size with a utility knife. Attach the weather stripping to the bottom of the door with staples, or nail it in place.
Paint the inside walls of your garage with bitumen paint. Bitumen paint is an all-purpose waterproof paint that will stop water from penetrating the interior walls of your garage. Apply one coat of paint to the interior walls of the garage. Allow to dry, then apply a second coat for maximum protection.
Clear debris from any storm drains in your yard. Clogged storm drains will often cause water to back up and enter the garage. Remove any leaves, sticks or other debris covering the drains.