You would have to get 80 reputation, then ask Moonfall to tell you the story of Mosskit, go to the Fireclan border, there should be an X on the ground. Head to the warriors den, take Darkclaw's mossball and put it on the X, a cat should appear after a while, which would be Moss Shadow. Bring Moss Shadow too Bramblestar and you become leader, if you don't, check if you have 80 reputation.