Just change my thermostat but my heater doesn't get hot what can I do to get it fixed

I don't know much about your year vehicle but I can give some ideas. I'll need more information before giving you my personal suggestion. Did this happen before installing the new thermostat? Are you using the factory recommended operating temperature thermostat? Does your vehicle warm up to operating temperature? IF not, "the new thermostat could be sticking open." Is there air in the system, "Did you leave the cap off the radiator to purge the air from the system till coolant started to overflow from radiator? Heater core could be plugged but unlikely. You can check that by removing the two hoses that go to the core and flushing it with a garden hose. Another suggestion would be to turn the knob onto the hot position. LOL just joking. Run a code Test to see if your getting any codes. Auto Zone or Oreily's Auto can should be able to help you.