1. check coolant level.
2. If coolant level is ok, check and make sure you car is in fact heating up properly and staying at temp. if it is not heating up or taking too long to heat up it is most likely your thermostat
3. If you have coolant and proper temp on your gauge it could either be an air bubble in your heater core or something plugging your heater core or radiator. check your upper and lower radiator hoses and make sure that if there is a temp difference it isnt something like 150 inlet and rediculously cold on the outlet. same theory goes for your heater hoses.
4. check for flow of coolant, pull your radiator cap and turn on your car and see if you can see coolant flowing thru it. if not you have a line plugged somewhere.
..sorry a lot of info, hopefully you can follow my garbled hints and good luck. questions feel free to pm me, i get a lot of this concern at work and tho i work on fords the same principles apply to all cars.