If your heater is not blowing cold, you can focus on two separate systems.
1) Is the engine itself getting up to the proper operating temperature? If the termperature is not hot enough, heat will not blow. Also, you could have a restriction in the heater core. Check the inlet and outlet hoses for how hot they get. They should both be very hot. Be careful of course when checking this.
2) You could have a temperature blend issue inside the car, which keeps the cold air from getting shut out of the ventilation system when calling for just heat. If the blend door motor does not work, it will not close the cold side and open the hot side (which happens at the same time on the same door). This motor failure is fairly common.
Look at your engine temperature first, then look on the inside. Good luck!