One example that you might look at to get you started is located
here.To use pattern matching, you will need something on your target that will not change size, this may not be practical for a robot that will be chasing something. You can also look at thresholding followed by blob analysis to determine which item in your field of view is the target. This method works if your object will not be the same color as the background. This is faster than pattern matching, and is more size independant.
