A doctor would prescribe the appropriate cream or ointment, antibiotic, antifungal
cream or a combination of antifungal and antibacterial
cream, depending what was causing the problem. Sometimes antihistamine medications can help if this caused by an allergic reaction to something that you touched or bug bites.
If the patchy skin makes you feel embarrassed, get some make-up to match the your skin tone to mask the white marks.