I have a white patches(pityriasis) on my back.i would like to know how to get rid of it?

You need: antifungal/steroid cream like lotrisone applied twice a day. I think oral diflucan (fluconazole) might also work.

Tinea versicolor?: If this is the actual lesion, which is very common, it's easy to manage. Physicians who have it take a big ketoconazole tablet once a month before they work up a sweat exercising, then don't shower for a few hours.