I can't believe you asked that question. Do you even know what the heater core does? All you need to do is go to Auto zone or whatever you got over there and pick it up. In most cars its behind the dash on the passenger side. Now for the overheating problem, it might be a different story. The heater core is only activated and takes anti freeze from the pump when you turn the heat on in your car.
Now the question stands. Are you overheating when you don't have the heat on and just drive around? Please decribe the overheating problem more.