You can always start by contributing to projects you know and like.

every project has some instructions on how to contribute and what to look at when doing first contributions.

If you are interesting in application server itself, note that it was renamed to WildFly and you can find some "getting starting" document at https://developer.jboss.org/wiki/HackingOnWildFly

but as I said, start by contributing to projects you know, usually on developer mailing lists if you ask about what you could look into, somewhere will point you into right direction.