Using Lego Mindstorms Inventions motors with arduino
I have done this quite easily with this shield. That may be a little overkill, but if you are like me and don't know much about voltages and stuff, it is a good safe way to get some good motor drivers. I power that off of my arduino with a 9-12v wall adapter.

In order to actually connect to the motors you have a couple of options. I split one of the lego wires in the middle and wired it into the motor shield directly. Another, less destructive way would be to solder two wires to one of the metal lined lego plates and connect those wires to the board.

The motors are really just simple dc motors, so nothing special is required. Just use the example code on the instruction page for that shield.