Dirty dandruff kitty?

Her name is Saki (yes like the Asian alcohol) and she weighs 32 pounds and she is so fat she can't clean herself. She has really bad dandruff and greasy hair and she sheds like crazy. I even used head and shoulders shampoo and it didn't do a thing. She is softer and not greasy after baths but the dandruff is just so bad. What do I do? (I'm only 12)