or just buy an rfid antenna, and arduino uno.
That would be like buying speakers and an antenna and expecting to listen to the radio.
Somewhere in the mix, you need something that converts the magnetic field (in the antenna) that the RFID tag disturbs to meaningful data, and you need something that creates the magnetic field in the first place.
That is not what the Arduino does.
But if you do want to use the Arduino to generate the raw signals it has been done before:
"This page is describes the construction of an RFID reader using only an Arduino, a hand-wound wire coil, and some assorted low cost common components."http://playground.arduino.cc/Main/DIYRFIDReader
Another good resource is Microchip Application Note "AN678: RFID Coil Design". It has useful tidbits like "The optimum radius for the transmitter coil is 1.414 times the desired range" and "for minimal loss, wire gauge numbers of greater than #26 should be avoided".