There is a clip on the wiper arm which presses against the knurled gear end of the wiper drive. You need to release that clip, then you can pull the arm up and off. It is best to go to the local auto parts store and buy a wiper arm removal tool for GM. Or you can probably get one cheaper on Ebay or Amazon.
http://www.autozone.com/test-scan-an...ol/439291_0_0/
You put the hook under the arm and pull the tool toward the arm - that hook releases the clip and you can pull the arm off. You can use a flat blade screwdriver to release the clip but I always scratch up the cowl vent when I do that.