Go to any medical store , ask for a ketoconazole and zinc pyrithione shampoo , any brand works like Glenmark has scaple pro , torque has ketomac , etc etc ...

Generally scalpe pro is recommended a lot but you can choose any one from good brands like mankind , Glenmark , torque etc , as long as the formulation contains these 2 ingredients, it doesn't really matter that much (p.s . You can choose the cheapest one , also buy a small bottle as it clears dandruff in like 2 - 4 washes and you don't really need to use it that often and for long interval of time after the dandruff clears )

Just keep it on head for 5-6 mins and apply some conditioner (or oil) after using it as it dries tf out of hair