Segmentation is a huge field of research. I would suggest looking in the literature about what you want to do, and use your favorite internet search engine to find implementations.
There are many ways to do a segmentation, thresholding, k-means clustering, and watershedding are the first that pop into my mind.
You could also have a look at the FEX entries by Image Analist. Over the years he has built up a sizable library of examples and demos.
