You need:
- keyswitches, keycaps, maybe stabilizers
- something to use as a plate (could be cut out of acrylic, metal, wood, ...)
- wire
- USB cable
- through-hole 1N4148 diodes
- resistors
- a microcontroller board (I recommend Teensy 2.0)
- soldering iron
- solder (get the leaded kind, either 63/37 or 60/40)
- flux
- ventilation (e.g. one of those solder fume extractor thingies, or a nearby window)
- some needle-nose pliers, some tweezers, and some tiny flush cutters (nail clippers work in a pinch)
Also helpful:
- multimeter
- panavise
- wire stripper
- nice desk lamp
- desoldering wick/braid
- solder sucker (e.g. Soldapullt)
- some dental picks
- heat-shrink tubing (and maybe a heat gun)
- hot glue gun
- jeweler’s loupe
Check out
http://geekhack.org/index.php?topic=42824.0