Lok at the various motor shields. there are several out there that have 2 H-Bridges that would allow you to control 2 motors. Most would require 2 Arduino pins for each motor (thats because they can also be used to power 1 stepper).
You need to know the current rating of your motors, and use that to get a unit big enough for your current requirements.
H-Bridge circuits are not simple, unfortunately they require 8 transistors and a few other components...
I know Sparkfun, and others have a Motor Shield for US$25 that is good for 2 amps. Uses an L298 H-Bridge IC. You can get a schematic from them also.http://www.sparkfun.com/products/9815