Start with making sure your clutch cable is adjusted properly. The clutch arm on the engine should have little to no play in it when the cable is adjusted properly, and this arm should be pointing more or less straight back to the rear of the bike.
With the right cover off, pull the clutch lever. The flower nut and clutch plate should both move away from the ring gear. If the flower nut moves, but not the plate, lock the clutch handle and tap on the plate or gently pry it up; it's probably just stuck.
If neither the flower not nor clutch plate moves at all when you pull the clutch handle, the clutch return spring inside the case is broken, or your clutch cable is very loose.
If they move a little, but not enough for the rear wheel to turn easily, your clutch cable is out of alignment.