USB-C cable plug it into what?

From the picture that looks like a USB-C plug, which means you can plug it into any USB-C charger.

Some (mostly Android) phones come with USB-C chargers.

Recent iPad models have USB-C ports. Use the charger that came with that if you have one.

Recent (<3 yr old) MacBooks come with USB-C ports for charging - use either the charger that came with the MacBook, or one of the ports on the MacBook itself.

If you're still out of luck, you should also be able to use a USB-A to USB-C cable - connect the USB-C end to the headphones and connect the USB-A side to any existing USB-A port (computer, wall plug, etc.)

Failing that you'll need to buy a USB-C charger such as any of these: https://www.amazon.com/usb-c-charger/s?k=usb+c+charger

No way to know with no details whatsoever.

Have you read the user guide for it to see what the cable is for?

What is the accessory in your picture exactly?

Going by the logo there, it looks like those are headphones. I would assume the cable is to charge headphones so would plug into a USB-C charge brick, but can't be sure.