Here are a few things you'll need to get started.
- Magnetic card reader (Mine is a Magetk 90mm dual-head reader. $5.00)
- AVR, Arduino, or clone (ATmega328p ~ $4.30 from Mouser.com
- solderless breadboard
- some wire
- maybe a header if you like that sorta thing.
- something to read your serial port. I use AVR Terminal from BattleDroids.net
That's all you should need to get started. Depending on the magcard reader you end up getting, you may have to modify these instructions, and most assuredly the code, to work with your specific reader. However, the code I've written should get you pretty far, I hope.