This would go in the confiuration.yaml
Added info: although over the past year a lot has been done to avoid this textual configuration and replace by GUI, still quite a few are left … also this will continue to apply for ‘templates’ which are needed if you want to do special things to / with your data.
So, at a minimum you would have to get comfortable with that … it will happen more often.
wrt to above, you can copy/paste it in, of course with your temp sensor and heater sensor replacing my examples. Important (!) is the indentation. climate: starts from column 0, the rest 2 spaces indent and seconds again 2 spaces. After that you need to restart HA and then hope it picks up.
As you are new, I also recommend to extend the logging section so you can see what is going on and not guess (too much).
Make sure your ‘logger’ section in the same configuration.yaml shows something below
logger:
default: warning
logs:
homeassistant.components.climate: debug