My heater won't get hot

There may be air in the system or the actuator valve for the heater core is not working. To bleed air out of the system;
Remove radiator cap when car is cool and turn the heater all the way to hot setting
Allow to reach operating temp and thjen top off radiator and overflow tank
Replace the radiator cap for good measure

The actuaor valve can be found on the inlet heater hose near the firewall. You can replace it by removing clamps and actuating cable or servo motor.