How to start on RFID?
1) An RFID antenna is a piece of analog circuitry and requires its own analog recieve/transmit circuitry. You can generally get modules that do this all for you and speak I2C/SPI/serial to your controller. That's by far the simplest way of doing this.

If you limit yourself to the 125kHz cards you can do it with just a GPIO pin (provided your controller is fast enough to PWM at that speed) and a bit of analog wiring: http://scanlime.org/2008/08/simplest-rfid-reader/ (note: requires good understanding and osciliscope)

2) Protocols, there's a lot: http://rfid.net/basics/186-iso-rfid-standards-a-complete-list-

3) The simplest transponders (cards, tags) just respond with their serial number, and you use that to decide whether to open the door. If you want cryptographic security, research MIFARE.