Does your car actually support CarPlay?
If so, does it support Wired or Wireless Carplay?
Consult your cars user guide, for how to set up CarPlay and if and what type of CarPlay it supports.
In general, if it only supoprts wired carplay, connecting a USB cable to the proper USB port on the car would enable Carplay.
If it supports wireless car play, there's generally a voice button on the steering wheel to enable it.
What make and model of car do you have?
How are you connecting the iPhone to the Car?
click here ➜ Use CarPlay with your iPhone - Apple Support