Cannot unscrew stuck shower arm
You need more leverage. To do that use a pipe wrench with a cheater bar attached so the handle is longer. Slip a 1 inch (or comparable) diameter pipe length onto the handle. This will greatly increase the torque you exert. Counterclockwise direction is loose.
Apply force slowly and in increments. Most likely there's a sweated copper fitting behind the shower arm and it can break if over torqued.
A more passive way to remove it would be with a hacksaw. Make a diagonal cut on top of the squared shoulder of the shower arm. Cut until you see the 1/2 inch nipple that it's threaded to. Make the cut from its base to the end of the square shoulder. It will spin right off when completed.