My heat blow cold some body told me i need resistor on a 91 Buick regal unlimited where do i find it

The resister only changes the fan speed
if you have not heat
first check coolant level if it's down then you have a leak that will cause no heat if it's ok
then run the vehicle and make sure the engine temp gauge reads about half if not change the thermostat if the engine temp is ok
then grab the heater hoses see if they are hot then the heater core is plugged and if the heater hoses are hot
then there is a problem with the heater control head
hopr this helps