how do you plug usb on new Apple Mac air

If you are referring to a USB-A, then you will need a USB-C to USB-A adapter, or a hub with a USB-C cable, and USB-A ports on it. There are less expensive, USB-C hubs available than the linked Belkin example.

Me-SA wrote:

how answer plz

Use an adapter USBC to USB.

Apple branded USB-C to USB Adapter - Apple

I've been using this one from Amazon:

From Amazon UK

From Amazon USA