-- 2 motors 2 directions

[system]

Hi

You need something called an "H-Bridge", to chage the polarity of the motor according to signals from the Arduino.

Here's a good explanation of one:
http://www.bobblick.com/techref/projects/hbridge/hbridge.html

You could also build one with a relay- very simple but noisy and won't last forever due to electrical corrosion since the contacts "spark".

Here's a commercial H-Bridge that I use often. It drives two motors, and is reversible:
http://www.hvwtech.com/products_view.asp?CatID=113&SubCatID=146&SubSubCatID=0&ProductID=30

Hope this helps

D