Good places to start are the todo forum and the bug tracker.
If you have access to a database with an xa driver writing a vendor-specific jca wrapper would be both fairly easy and useful.
Most people start by fixing a problem that is in their way or implementing a feature they need.
I wanted to know the entire structure of the Jboss,by participating in the froums you get an overview only.I wanted to know how the modules for jboss have been designed.
look at sourceforge