Read Bicycling's repair manual or Lennard Zinn's book and consult Sheldon Brown's and Park Tool's web sites for background info. Wheel truing isn't difficult but you should have a good concept of how wheels are constructed before beginning.

Park Tools sells loop-type spoke wrenches is a variety of sizes and they are only about $6 each. The "black" wrench (SW-0) fits DT, Wheelsmith and most other European spokes and is the most commonly used one on good quality modern wheels. The "green" wrench (SW-1)fits a few brands like Union but isn't commonly. needed. The "red" wrench (SW-2) fits older Japanede spokes. Park also makes and adjustable wrench that fits all three sizes but isn't a handy to use.

Take your wheels to your LBS and find which wrench fits. Buying all three is really a good idea as it will allow you to work on nearly anything you will come across except some of the new boutique Shimano, Campy and Mavic wheels.