I just bought an iphone 12 and it came with a USB-c to Lightning cable but no USB-c connecting block. What should I buy? Can it handle a 20W USB-C block?

The 20W is what I would buy: Buy 20W USB-C Power Adapter - Apple

It is compatible (see the Compatibility section of the above link).

Or if you have a USB-C power adapter for a Mac, you can use that. See https://support.apple.com/en-us/HT202105

Or if you have both the cable and the power adapter from a previous iPhone you can use them.

So since my old phone cord is so frayed that it no longer works, I need to buy something new on top of the price I paid for my new iPhone 12?

How disappointing