[Gdeaver]

Sharp makes a IR distance sensor that is good out to 5 feet. It has a narrow cone of detection. Ultrasonics can sense farther out but have a wide cone of detection. The outdoor environment is a nasty place for IR and ultrasonics. You might consider starting to experiment indoors in a small room. A couple years ago I work with some students on something like this. We built a Vex square bot and mounted a Sharp IR on a servo. The Sharp IR is a simple A to D device. If your ADC readings have allot of bounce, try soldering a 100uf cap across the 5+ and ground at the sensor. Basically you move the servo take an IR reading move again take a reading. Remember it takes time to move the servo so you need a little delay before taking a sample. With this you can make a simple object avoidance robot. After you get this down you could start to play with environment mapping. Add an encoder to measure how far the robot goes and relate that to the sensor data. Its a case of banging a matrix. Take little steps. Tackling the outdoors maybe a little much at first.