How do you repair the heating system?

The question is a bit vague and very general - as the halogram used to say on Star Trek, state the nature of your emergency? Are you looking for trouble shooting info or how do I replace/repair something? What is the engine temp? does the cooling fan run all the time, do you have a bad heater, heater switch doesn't work, what is the thermostat doing. Did someone steal your heater?