I HAVE NO HEAT UNTILL I REV THE MOTOR ON MY HYUNDAI ELANTRA GT

is the thermostat that is in it, the right temperature thermostat. Had the same issue with my jeep from the thermostat the parts guy gave me being a lower temp one?

If the T-stat check out good, then possibly an air lock. I assume the coolant is at the proper level. But, if it got low, then was filled, it may have left a bubble. Do the following, make sure the engine is cooled down well enough and remove the radiator cap. Then run the car and bring it up to temperature. Watch the coolant level (don't look straight down the radiator opening, might spew in your face). Level should be above the fins. Rev the engine a little continuously for a minute or so and watch if the coolant level drops to where air gets into the fins. If this happens, add coolant then.