It could be your heater core that needs to be replaced, or the thermostat which is easy and inexpensive especially if you doit your self, and don't forget to check your radiator for leaks. Also check the radiator cap, check to see if the rubber seal is dried and cracked

Please see my tip at http://www.fixya.com/cars/r5929960-s_thermostat . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.