How can I create a simple steering mechanism for a car?
The most common I've seen is rack and pinion:



Essentially you have two parallel beams with one fixed to the car's chassis. The other bar moves horizontally which changes the direction of the wheels. Attach some gears and you can hook it up to a steering wheel.

The steering technique has been used in the Whirl N' Wheel Super Truck (5590)
 set. The instructions for the set show how an similar rack and pinion mechanism.