If you are straightening it you are using heat. My guess is you are blow drying as well, which is more heat. Look for a heat protectant that you put in after towel drying. It will seal and protect your hair.
Also do not rub your hair with a towel when drying. Work the towel from the roots to the ends. Your hair is actually made up of scales and rubbing back and forth with lift the scales making it frizzy. Also never brush wet hair. It will tear the hair. Use a wide tooth comb instead.