If you use the NXT program for education, "go on the line" has been introduced by "Robot Educator".
This is the program of line tracing robot.

Here is the program I made to imitate "Robot Educator".
[DOWNLOAD] Created Program 1(for the NXT software)

"Switch (branch instruction) "is set to "light sensor". I think the sensor values is the best "47" to compare with the middle range of 32 to 62.

I tried to run the program. Robot moved willingly on the line. Well done!! However, the movement has been knocking a little.

When you figure the movement of the robot, it would look like this.
The arrow is the direction of the robot proceeds.
The robot is progressing in the boundaries of the "black" and "white" like a zigzag pattern.
Red arrow is the "detect the black" and blue arrow is the "detection of white".