First, you need at least 80 points, then once you become a warrior, you go talk to the grey cat at the herb grounds, after that, go to the warrior den and take the moss on one of the beds. Go to the beach and there should be an X, place the moss there. It should take 1 or 2 days but then Mosskit will appear, talk to her, take her to Bramblestar and then you'll be leader