1990 Chevy Beretta Heater Core not hot

First thing to check is if your coolant is low or not. That is the number 1 problem with no heat. You may have changed the core an dnot filled it correctly. It could also have an air lock in it. There is a bleeder valve by the passenger side front to bleed off the air. If you hav to add quit a bit of coolant then take it to a pro and let him pressure check it for leaks. It's leaking someplaceif it's low.