There are a number of ways to connect RFID readers to Raspberry Pi (and Arduino-style) devices, but it all boils down to interfaces.
The simplest interface for most of us is an RS-232 serial interface, and most of the pre-packaged readers use this, I think. The Pi doesn't have old-time PC serial and parallel ports, but a USB/RS-232 adapter is relatively inexpensive.
There are other types of interfaces as well, that can connect to the Pi's GPIO pins, but unless you do that sort of stuff for a living like me or you got a good deal on a surplus
unit and want to save money, stick to the serial port option.
The actual output of RFID readers is usually just plain ASCII text, often nothing more than a verbatim copy of the data that is stored on the RFID tag or card.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.