Alright. If you are a beginner and wish to learn usb interfacing with a microcontroller, then I would suggest you to start with Arduino board.
I hope the following link will help you regarding the same.
http://petrockblog.wordpress.com/2012/05/19/usb-keyboard-with-arduino-and-v-usb-library-an-example/
http://www.practicalarduino.com/projects/ps2-keyboard-or-mouse