RFID from "fist principles" is quite complex for 1st time uses of microcontrollers and 1st experince with RF or RFID.

You might be better pff purchasing an RFID module and interfacing via a UART/serial port of the microcontroller.

One example is here
http://www.adilam.com.au/RFID/rfid.html

If you still want build an RFID reader from "first principles" then you have a lot of research and experiments to do before creating a finished RFID product.... even if you have the C code to talk to the reader IC HTRC110.

The European magazine Elektor had some articles in 2006 and 2005, that might be a start.

Joe