you need to be a warrior, find moss-shadow by talking to moonheart and place the mossball from the warriors den at the "X" at the beach. wait untill moonhigh, and moss-shadow should appear. do as she says, and once you have 80 reputation, bramblestar will make you leader. you need all five gems to activate the cold stone, the one in the river at "the dip", the gem nightwhisper found, liontail's gem, the one in the tree with alyssa and the one in the river at the fireclan border. click the big stone in the gem cave, and you'll get to the cold stone. btw if you don't go to fireclan and defeat firestar quickly after that, it will start raining and everyone except you will die in brambleclan.