how to use a USB A on a MAC

On a new one? With one of these -->

USB-C - USB adapter http://store.apple.com/us/product/MJ1M2AM/A/usb-c-to-usb-adapter

Of course Apple as usual doesn't say to which US speed standard their cable is rated. The Amazon adapter is USB 3.0 which is certainly more than adequate for something like a mouse, but it isn't up to 3.1 or 3.2 speeds which could be 2 or 4 times faster transfer speed than the Amazon adapter offers. Of course it also depends upon the speed of whatever it is you are plugging into the adapter.

Or 3rd party versions like this one from Amazon (I have a handful of them).