You control the temperature in your room with the thermostatic knob on the radiator. If you set it to position 3, the temperature in your room will be around 20 ºC.

You can also control the temperature with the room thermostat hanging in the kitchen, just make sure the radiator(s) are open.

Are you going away for a longer period of time?
Then set your heating to a lower setting, around 15 ºC is a good option, so it doesn't cool down too much. That way you won't burden the environment unnecessarily.