Have you read the documentation that comes with
Tomcat? It is really very extensive - a normal Tomcat installation has it as servable web pages under tomcat-docs. You can also study the way the example "servlet-examples" web application is set up - very informative.
You should also study the
servlet API, downloadable
from here. If there is something you don't understand after reading this material, by all means post about it.
Bill