first, become a warrior, make sure you have 80 rep, then talk to the gray cat in the herb field. Once she tells you about moss kit grab the moss ball in the warriors den and put it on the X near fireclan, keep checking there until you see her, bring her to bramble star and he’ll make you leader! :D
You have to have at least 80 rep and find mossshadow. To do that, talk to Moonheart when you are warrior, then go to warrior den, take the moss ad put it on the red x on shore and wait.