Hi there. I have recently designed an RFID reader and been through a development process trying a number of different ICs. I can't give you schematics or code but I can point you in the right direction:
There are two very good ICs to use:
1) Atmel U2270B - available from www.atmel.com
(buy from www.farnell.co.uk
2) EM4095 - available from www.emmicroelectronic.com
Both of these are 125kHz passive read/write ICs that act as a front end for your reader. Simply follow the design guidelines on the Atmel or EM websites and they show you how to create a circuit that will produce an encoded output from a tag with minimum components.
This data stream from the above devices must be decoded to obtain the actual tag serial number. You will need to obtain the datasheet from the RFID tag you want to use to find out the encoding type (i.e. Manchester encoding, PSK) and data rate and then use a microcontroller to decode the bitstream and create an interface (RS232, USB, CANbus, etc.) to send the serial number out on.
Hope this helps.