To prepare an SD card for your computer, you will need:
A 'regular' computer with a built-in SD card reader, or an add-on USB SD card reader. The instructions here are for Windows and Mac. You can pick up a tiny microSD card reader/writer that is nice and fast and works with all USB ports at the Adafruit shop.
An SD card at least 4GB in capacity. You can pick one up that is tested and guaranteed to work with the Raspberry Pi at the Adafruit shop. (If buying elsewhere, remember that for the Pi 2, you'll need microSD.)