Grab keys from an ordinary USB keyboard
As suggested in the comments, you have two options:

As suggested by Majenko, you can use the Arduino USB Host Shield:


Alternatively you could use a PS2 keyboard instead (as suggested by Gerben). Arduino already has a library to handle PS2 keyboards, so the implementation of it would be easy. For reference (and example program)
playground.arduino.cc/Main/PS2Keyboard

If you do not have access to a PS2 keyboard or just want to use a USB keyboard, Gerben further suggests that most USB keyboards can be converted to PS2 keyboards with an adapter:


The keyboard must be able to support both formats (USB &amp; PS2).

The following webpage shows the connections needed to go from USB to PS2:  USB to PS/2 adapter pinout