A typical Moen shower does not have any nut to unscrew. Under the handle, if you see a sleeve with a "bump-out" on it, pull that sleeve out. Then you will see a tiny hole in the top of a horshoe clip. Turn off the water, and pull that clip out. It is what secures the cartridge in the body. You may then need special moen cart. puller to rotate and loosen the cartridge before pulling on it. Post a picture here for more details.