You have to have your heater on or your car overheats what might be wrong?

It could be any number of things, but here's what to check...

Make sure the radiator is full of coolant (with the engine off and ice cold, facing slightly uphill)

If you need to add coolant start the car (with radiator cap removed) and add coolant while its running directly to the radiator. Once the radiator is full replace the cap then add coolant to the reservoir.

If coolant is not an issue, then make sure the fan is coming on when the car is trying to overheat...a dead fan or bad solenoid to make the fan run is very common on newer cars, older cars used a directly attached fan that always ran with the engine on.

Next thing to check is the termostat, if the car starts to overheat and the top radiator hose is hot and obviously coolant isn't circulating through it then a bad t-stat is to blame.

It could also be a bad water pump; however, under normal circumstances it would leak a small amount of water from a "weep hole", the location of the water pump varies slightly with vehicle.

Lastly you could have a blockage in the radiator requiring repair from a reputable shop or replacement (which is usually cheaper, if you're the one doing it).

Good luck!