how do I fix the heater which is only blowing cold air

Ok, this is going to sound really stupid, but I brought the car in the spring and it was already hot so I only checked the air conditioner and when that worked, I assumed the heater worked as well

have you had the dash apart? if so or if you do most hvac controls are either vaccum or electronicly controlled. if the control knob is realy not connected to anything look for a loose connection floating around in the dash that matches the back of the control knob.