Try making a black line on a light-colored floor with black electrical tape, then start the robot facing perpendicular to the line at any distance away.  It should stop right at the line.

The Light Sensor outputs a number from 0 to 100 that indicates the brightness of the light seen by the sensor (0 is darkest, 100 is brightest).  In order to test for a "dark" color, the Wait for Light Sensor block specifies a brightness value to compare against and a comparison direction (< or >) in the Light field.  In this example, the test "< 50" is used, but you may need to modify the 50 value for your conditions, or calibrate the light sensor to adjust its readings (see below).

Line Sensor
(Substitute
Light Sensor)

The H2-DispLight program is a utility program that simply displays the current reading from the Light Sensor on the NXT screen and keeps updating it, so you can use this program to determine what brightness values to expect from your robot in different circumstances. 

The H2-DispLight program uses the DisplayNum My Block, which provides a convenient way to display a number on the NXT screen with a label in front, which is useful in a variety of different situations.

As an alternative to measuring brightness values and modifying your programs for different conditions, the NXT software includes a tool to calibrate the Light Sensor, which is a way to adjust its readings so that the minimum expected brightness will be output as 0, and the maximum expected brightness will be output as 100.  After calibrating, you can be sure that, for example, the brightness test of "< 50" in the H1-FindLine program will be True over the line but False over the open floor, without needing to adjust the 50 value in the program for different lighting or floor conditions.

To the use the NXT calibration tool, in the Tools menu, pick Calibrate Sensors, then select Light Sensor and the correct port on the NXT (3 here).  The software will download a program named Calibrate to the NXT (and will also run it right away).  Take the robot to the surface to be used and place it all the way on the floor as if it were driving (don't hold it off the floor in your hands, that will change the readings).  Follow the prompts in the Calibrate program to sample the Min (darkest, typically with the sensor centered over the black line) and Max (lightest, typically over open floor) brightness values.

After using the Calibrate program, you should be able to use the H2-DispLight program to verify that the sensor reads 0 (or close to 0) directly over the black line, and close to 100 over the open floor.  You can then go on to test the calibrated brightness of other colors or areas on the surface for more detailed tests in your programs.

Note that the H2-DispLight program always displays calibrated brightness values, which are the same values that your programs will get from the Light Sensor.  In contrast, the View Reflected Light feature in the NXT brick menu displays un-calibrated brightness, which is not as useful.

Line Sensor
(Substitute
Light Sensor)

Now if you make a large black square (or any shape) with electrical tape on a light-colored floor and start Multi-Bot inside the box, it will try to drive around inside the box "bouncing off" the borders and staying inside. 

Staying inside an area marked with a colored border is the first step in playing a "Robot Sumo" game, where two robots try to push each other outside of the "ring".  You can play a simple practice Robot Sumo game with one Multi-Bot robot by placing various objects inside the ring with the robot, and see if Multi-Bot with an attachment such as the Sumo Pusher can push them all outside of the ring while staying inside itself.  Try changing the time of the back up and the angle of the turn in the LeftPivotAngle block to get different driving patterns and see which work best.

Line Sensor
(Substitute
Light Sensor)

Sumo Pusher
(Optional)

Note that in this kind of line following, the robot is actually following the right edge of the line, not down the middle of the line as you might expect.  The robot tries to keep the black line just to the left of the sensor and the open floor to the right of the sensor, by zigzagging back and forth on either side of the edge, alternately seeing the line and the floor.

Line Sensor
(Substitute
Light Sensor)

The H4-LineFollow2 program is an example of a "Two State" line follower, because it chooses from two different motor settings (turn left or turn right) depending on the brightness seen from the sensor.  A limitation of a two state line follower is that it can only do one kind of turn in each direction.  If you make the turns gradual, it will drive smoothly but not be able to follow tight turns in the line.  If you make the turns tight, it will be able to follow tight turns in the line but will drive slowly and zigzag a lot even when the line is straight.

The "Four State" strategy in H5-LineFollow4 will choose between four different kinds of turns (sharp right, gradual right, gradual left, sharp left) depending on the sensor value.  This will allow it to use the gradual turns when close to the line edge (when the line is mostly straight) but switch to the tight turns when the sensor falls away from the edge, as will happen for tight turns in the line.

In order to divide the sensor values into four useful ranges, it is necessary to know the exact range of values expected, so it is important to calibrate the Light Sensor first.  When the sensor values are known to be from 0 to 100, the program can easily assign a tight turn to the last 10% of the top and bottom of the range, but use gradual turns elsewhere.

You can try adjusting the motor speeds used in the four different kinds of turns used in H5-LineFollow4 to change the response to suit different robots and different courses.

Line Sensor
(Substitute
Light Sensor)

Thinking about how the "Four State" line follower in H5-LineFollow4 works, can you imagine an "Eight State" approach with four different degrees of turn tightness in each direction?  How about even more states?  Programming each of these states individually would be very tedious.  Fortunately, we can come up with a mathematical relationship between the sensor reading and the motor power desired and simply calculate the motor power based on the sensor reading directly, without the need for any Switch blocks to separate the cases.

In the H6-ProFollow program, Math blocks are used to calculate a motor response for each possible sensor reading, so that the amount of turning is proportional to the approximate distance that the sensor is away from the edge of the line that it is following.  The farther away from center, the tighter the turn.  The approximate distance from the center of the line is calculated as the difference between the expected sensor reading at the center (50) and the actual sensor reading.

Understanding the math behind this approach may be harder than understanding the H5-LineFollow4 program, but note that the resulting program is actually simpler in some ways, and it should result in much better line following.

Line Sensor
(Substitute
Light Sensor)