Ultrasonic Rangefinder

An ultrasonic range finder sensor enables a robot to detect obstacles in its path by utilizing the propagation of high-frequency sound waves. The sensor emits a 40kHz sound wave, which bounces off a reflective surface and returns to the sensor. Then, using the amount of time it takes for the wave to return to the sensor, the distance to the object can be computed.

The sensor can be used to determine distances to objects. It can be used as a tool to determine if any objects are in the robot’s path at all. To increase the sensing range, the sensor can be mounted to a servo to allow it to rotate. The following are some basic information to help you kick off your project. 

Documentation: http://www.vexforum.com/wiki/index.php/Ultrasonic_Range_Finder

Instruction: http://content.vexrobotics.com/docs/instructions/276-2155-instr-0312.pdf
Video: http://www.education.rec.ri.cmu.edu/products/teaching_robotc_cortex/
then click Sensing and select Forward until Near.