Have you bought a car radio with Carplay support, and do you want to use it? Then connect the USB cable of your Iphone to the supplied USB cable of the car radio. Usually you get 2 USB cables with the radio, and only 1 of them works with Carplay. Usually you will find a label on the USB cable with "Carplay" or "Mobile".
After connecting, Carplay will immediately appear on the screen of the car radio.