My thermostat is clicking, but the heat is not kicking on. The thermostat reads -AC where the temperature is.

You may not be getting 24 volts AC to the thermostat. Remove the thermostat & check for 24 volts between red & another wire. If you get 24 volts (+/-) install a jumper between red & white. If the furnace then comes on, the thermostat is bad.