<p>I need to build full-text search functionality into our knowledge base system using C#. How do I start?  </p>

For example from Lucene:
http://lucene.apache.org/java/docs/
C# port is here
http://incubator.apache.org/lucene.net/
You should also read something about the the information retrieval, and you can start with this one :
http://www.amazon.com/Modern-Information-Retrieval-Ricardo-Baeza-Yates/dp/020139829X