how do I charge an IPhone 12 ?

If you have an older power adapter and cable from a previous iPhone or iPad you can use that. If you need to buy a new power adapter to go with the new phone and the cable that came with it, see: https://www.apple.com/shop/product/MHJA3AM/A/20w-usb-c-power-adapter

Also see the following from https://www.apple.com/shop/buy-iphone/iphone-12

As part of our efforts to reach our environmental goals, iPhone 12 and iPhone 12 mini do not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.

We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones which are compatible with these iPhone models. But if you need any new Apple power adapters or headphones, they are available for purchase.