Squirrels are important to the propagation of trees, so even though they might be pests in your garage, it's considerate when you try to remove them in a humane way. However, you do have to be cautious around mother squirrels, as they can become aggressive and protective over their young.
Hang sweaty clothes, play loud music or leave the lights on to encourage the squirrels to leave the garage on their own. Another option is to create a model of an owl and use it as a scarecrow.
Block holes with crushed newspaper and spread flour near the squirrels' nesting area. If you see footprints or notice the paper has been forced out of the hole over the next couple of days, you will know that it's time to use a cage trap.
Bait two to three cage traps made for squirrels and place them in your garage near the nesting area. Once the squirrel(s) have been captured, release them back into the wild (preferably 15 to 20 miles away in a wooded area).
Make necessary repairs to your garage immediately to prevent more squirrels or other small animals from trespassing. Check for loose shingles or holes in the garage and also trim any trees that hang over your house.
If you suspect there are abandoned young squirrels in your garage or that the squirrels are rabid, contact your local wildlife center or animal services so they can properly take care of the situation.