Extreme case of Dandruff!! ='(?

iv tried everything!! and i dont know what to try now, ive used some stuff the doctors gave me in the past but it just dosnt go!! does anybody know the causes of it?? i have very thick curly hair aswell btw and i get dandruff whether i use hair products or not, so i dont know whether iv just got a dry scalp or what, i would appreciate all ur advicee!! thanksss xxxxxx