Assuming you're not the original owner of the car, has the heater ever worked for you?
Does the car's dashboard engine temperature gauge move up to partway between C and H? If it stays near C, you may have a stuck engine thermostat.
Under the hood, there should be a supply and return engine coolant hose running to the cabin firewall. There should be a valve box on one of them with a pull cable (the other end of the cable is your cabin temperature slider control). Does this look OK?