Data Clustering is one of the excellent method for discovering and visualizing groups of stuff in the Data mining field. It is one of the Unsupervised learning method. Though it is a vast topics I would recommend you to read Chapter 3 of
Programming Collective Intelligence[
^] book(if possible). This book would help you a lot in regards to this topics.
Note: The example this book used based on Python language.
Anyway, beside that book please have a look few example of Data Clustering using C#,
Hope it help you a bit to get to the point :)