[EHaskins]

There are two ways to do line following with an RCX/NXT.
The first easier, but slower way is simply if it sees black go left, if it see white go right. This approach uses only one light sensor so if you want to use the other sensor for something else you could.

The second more complicated way is to have two light sensors mounted (in your case) about 1 1/2-2'' apart. When the left sensor sees black slow the left motor, and increase speed of the right motor, and when the right sensor sees black slow the right motor, and increase speed of the left motor. This approach is more complicated, but the robot will move faster.