sometimes dandruff is caused by (just) dried or overdried scalp or local irritation mao na mag-pakpak...
most of the times, this is caused by a fungal infection...
sa first, just regularly shower, use a conditioning shampoo, have a hot oil treatment at least once a week and as much as possible avoid strong chemicals like gel sa imo hair...
sa second, you need a good anti-fungal like ketoconazole (nizoral)...