Nothing short of a mechanical actuator of some sort will "press a key"; perhaps you mean "send a keystroke to my application"?
You need to supply more detail; the Arduino alone can't send a keystroke - you could send a character down the USB serial to the PC, where software there could interpret it and then perform the action to send a keystroke (depending on your operating system, and the language + libraries you use there, this may or may not be a straightforward task).
Are you expecting an Arduino to send this request, or are you interested in something to do it under Processing? Java? Some other language? Many missing details are needed...