SirFizX

Added the resources to the tip.
Regards,
Happy New Year!

Here is a fine book on UML by two of the UML's original authors.
http://tinyurl.com/uml-textbook

Here is an experimental online uml diagramming tool where you can create quick diagrams to save your ideas or post them to the web.
http://yuml.me/diagram/scruffy/class/draw

Here's are two nice discussions of class diagrams.
http://tinyurl.com/umlclassdiagram
http://www.agilemodeling.com/artifacts/classDiagram.htm

Here is a nice free UML Modeling Tool.
http://argouml.tigris.org/

This introductory tutorial is broken into nice learning chunks with good illustrations.
http://www.cragsystems.co.uk/uml_tutorial_download.htm

This paper provides a nice overview of uml diagrams and is still a very useful learning tool even though it references an older version of UML.
http://tinyurl.com/uml-intro