First, make sure the coolant is full and clean. Next, make sure the engine is running at a normal operating temp. If it isn't warming up, replace the thermostat. Finally, if none of that helps, start the engine, let it warm up with the heater on high. Feel both heater core hoses. If they are both hot, then there is coolant circulating through it. In that case, check the blend air door. On the other hand, if one heater core hose is hot and the other slightly warm, have the heater core flushed.