StarUML is a good tool, I started with that. While doing sequence diagram I could not find certain UML features (They are there e.g. asynchronous message and return message).
So, I exported XMI file and created the other diagrams on Sparx Enterprise Architect.
I suggest using StarUML for new users to UML as it is simple to use and has smaller learning curve.
Original domain model in assignment had about 10-12 classes. My class diagram had about 40-45 classes.
Class diagram included JSP pages as well.
Component diagram, I followed Mark Cade book approach. Combined certain JSP pages as single component e.g. certain request form and corresponding response page was combined as single component.
Component diagram included tiers and component in tiers.
To quickly gain some knowledge on UML diagram, you can go through http://www.uml-diagrams.org/