Where is the heater core and how do i replace it on a 2000 Ford Focus?

It is usually in the heater housing inside the dash. I'm assuming you are not familiar with vehicle repairs. It is not that complicated to replace but it usually requires the removal of the dash. I have never done one on a focus, i've only done them on mustangs. If you do not have coolant on the passenger side floor board or when you turn your heat on if it does not immediately fog up your windows i see no reason to replace it. These are the usual signs of a bad heater core. If you are just not getting heat in the passenger compartment you could have other problems. A chiltons manual will give you some instruction on how to replace it. Good Luck!