Your first real robot the easy way, learn all the basics while at it :)

Step 11: 1 Sharp Analog Infrared Range Finding System (AERS) with cable

The one "eye" sends infra red light. The other sees the reflection of this (if there is one), and the unit returns the distance to the object in front of it. It has 3 wires (make sure you get the cable for it, or it can he hard to hook up). You give it power on 2 of the wires, and the third one plugs into the microcontroller, and tells it the distance.