Woops, sorry, stupid... Never posted something before here (surprise).
I'm using a RFID-RC522. Furthermore, I don't use an Arduino, but a Teensy 3.2.
Jep, I managed to read an ID from the cards and display it on the Serial monitor. The library 'MFRC522' has some examples, one of them is reading the NUID.
Also, I have a code that should work in the way I want to. However, when I connect it, the serial monitor shows squares (like undefined symbols) that appear in a row.
I attached the code to this reply. Maybe, you cannot run it since you're missing the required library. Through this link: http://www.instructables.com/id/Arduino-RFID-Reader-MFRC522-Turorial/ you can find a zip file, that you should use to install the library.
The array 'cards' contains a list of numbers that are connected to one (in this case out of 5) cards. In the void loop is checked whether the scanned card contains a list which corresponds to one of the lists inside 'cards'. If so, it writes the number of the place inside the array 'cards' to the serial port. If not, it writes '6' to the serial port.
Code_Armando.ino (1.2 KB)