1) Check your thermostat to make sure that it is opening up.
2) Once the car has warmed up grab the lower radiator hose and see if it is hot. If its cold then coolant isn't being transfered from the engine to the radiator.
3) Could also be a blockage in the heater core.

I would go ahead and flush the whole system with a good radiator flush and add 50/50 coolant/distilled water.