What kind of extension cord should I use in a garage?

Use a heavy-duty outdoor extension cord, not a cheap indoor one. Garage floors are hostile—oil, chemicals, and vehicle tires can damage a flimsy cord. Get one rated for outdoor use with a thick, oil-resistant jacket (look for SJTW marking). 12-gauge is safer than 14-gauge for longer runs or tools. Keep it off the floor when possible, and use a cord protector ramp if it must cross the floor.