This is a fairly straightforward guide of how to build a line following robot. it uses lights, light dependent resistors (LDRs), an arduino, and a boe bot chassis (any robot chassis with 2 continuous rotation servos will work). it reads the sensors and turns based on the amount of light reflected form the surface below it. the sensors could also be used to detect edges.
Step 1: Build the sensor
you will have to build the sensor to the included schematic
Step 2: Mount the arduino, and connect the wires
i used an arduino mega, but you could use an uno. mounting will vary based on arduino and chassis. you could also use an arduino board of education from parallax.