What to do about an itchy scalp?

My head itches constantly. I have no lice, I checked just a few days ago. But my head has itched for months! I scratch my head a lot and I have some dandruff. It feels so irritated, especially at the back of my head. Sometimes I scratch it so much it hurts. Could it be my shampoo? I use Herbal Essences. My hair gets oily too, and if I ever don't get a shower one day, the next day it itches a lot. The water in my shower is hard water and has a lot of pressure. I always let my hair air dry. After I get out of the shower I get the excess water off with a towel then let it dry naturally. I usually go to bed with my hair still kind of wet. Could a new shampoo, maybe Selsun Blue work?