I don't have dandruff but i am suffering from hair loss . why?

Depends on cause...: One's primary care doctor can check for medical causes. If none are found, then one can see a dermatologist, especially one specializing in hair restoration (some treatments: minoxidil, finasteride, hair transplants, ...). Medical causes may include: hormonal problems (low thyroid), seborrhea, alopecia areata, stressful illness. Other causes: genetic male/female pattern baldness, telogen effluvium.

See a doctor: You need a detailed evaluation. Cannot diagnose over the internet. Most common cause of hair loss is genetic, but unusual at 18 yo. Need to see you to diagnose.