What do i do about a bully..?

Well you can do the adult thing and either talk to her about it or find someone older to help, or you can go up to her and tell her she's only doing this as a means to force her own insecurities on others so that she can feel better about the fact that she can't cope with who she is. Also there's a decent chance that she may hit you for saying that, and if that's the case threaten ault charges.