I first looked at existing rovers that have been created using the Raspberry Pi, and what electronic parts they used.
These two projects seemed to offer the most information:
It seems that I can either wire through a motor driver board, or simply use a breadboard and resistors.
Here is a diagram of the Raspberry Rover which uses a motor driver board (click to enlarge):
The board takes the signal from the GPIOs and controls the voltage directly so that there is no need for resistors. Both wheels on the same side are linked to the same set of 2 outputs. It is probable that this rover uses tank controls (to turn, wheels on one side reverse while on the other they continue forwards) as there is no servo to control the steering.
This is a diagram of the wiring on the Curiosity Pi (click to enlarge):
Instead of motors and a motor driver, this rover uses 16kohm and 180ohm resistors which are connected to servos. This controls steering, camera motion and the drive of a single front wheel. This method may be more complicated when more wheels need to be controlled.