Dandruff occurs due to excessive growth of an organism along with an oily scalp.
Use a ketoconazole containing anti-dandruff shampoo initially twice a week for a few months, an then atleast once a week.
You should maintain yourself on these shampoos an not stop using them or else dandruff tends to recur.
Avoid applying conditioners and gels to your scalp,you can apply them atleast an inch away.
Certain oral medications can be given only after having a proper look.
You can consult a dermatologist accordingly.