How to get keyboard input to arduino from the computer its plugged in to?
You can't. It's impossible.

Or not as you have requested it, anyway.

The Arduino can only talk over Serial, so you will have to have some software running on the PC that transfers data (however that data is generated) over serial to the Arduino to process.

The only other option will be to have one of the Arduino boards that is capable of acting as a USB Keyboard device, such as a Leonardo, plus either a PS/2 keyboard or a USB keyboard plus a USB Host shield. Your Arduino the receives the keystrokes from the keyboard, processes them, and sends the result on to the PC.