I have an Arduino Mega and 3 Nanos. Is there any way I could use these to make a usb-hid keyboard?
Forget the Nanos for this, the Mega might be used as it's USB coprocessor (ATmega16U2) can be re-programmed to act as a HID keyboard, although that's definitely no task for an Arduino beginner. I strongly recommend to buy a Leonardo (or Micro) as it's main processor (ATmega32U4) handles USB directly and you can program the HID functionality using standard IDE features.