I have just purchased an iPhone 12 how does the charger work

iPhone 12 does not come with a charger.. and it also comes with a USB C to lightning cable

If you don't have a USB C charger, then you'll need to get one.

Have you had an iPhone before? If so, you can use your previous cables..

Apple sells this

20W USB-C Power Adapter

or you could get one from Anker

https://us.anker.com/pages/uai2020

or just get a normal USB to lightning cable

Lightning to USB Cable (1 m)

or again, Anker makes em

https://www.anker.com/products/variant/powerline-iii-lightning-cable/A8813021

or you can get a wireless charger

If you have a USB-C power adapter, you can connect the included charging cable to it and charge the phone that way.

Otherwise any USB power adapter from another device (iPhone, iPad, iPod, Mac, Android, Windows etc..) will work with a Lightning to USB cable as well as plugging it into a USB port on a computer, TV, router, wall plate, car stereo, Wi-Fi router, game console, dvd / blue ray player, etc....