this is generally caused by air trapped in the heater core
or a blocked heater core
The best thing to do is pull the front of the car on ramps to elevate the front end.
place a coolant bucket in place of the radiator cap
and add some coolant
once the car warms up fully, rev the engine to 2500 RPM's and hold it for a minute
do this 3-4 times to get all the air out of the system.
then once the system is topped off and no longer bubbling. remove the coolant bucket and replace the cap.
when your reving the engine have the heater ON and coming out the vents so you can feel it
to see if your getting the trapped air out
if this doesn't work, you may need to back flush the heater core, to verify its not clogged
and if it turns out NOT to be either of those common things, the last resort would be a faulty air mix door
do you have any other questions?
if you need anything else let me know, if not please leave a kind rating.