check your coolant/anti-freeze level when the engine is cold. if you are low on fluid, fill it to the proper level. IF you were REALLY low, this may be the source of your problem. If all is good there, then:
after the vehicle has been running for awhile (10 minutes), from the engine compartment side, feel the hose leading into your heater and the hose leading out of it. they should both be warm/kinda hot to the touch. if not, either your heater core is blocked or the valve controlling the flow of the coolant is not working properly. if they are both warm, then your diverter doors and/or controller switch are suspect.
if your diverter doors are vacuum driven, check your vacuum lines in the engine compartment to make sure they have not been damaged.