In my opinion you need to read a little bit first : I suggest the "bible" i.e. "The essential client/server survival guide" by Orfali & Harkley.
It will give you the perspective you're lacking. By itself nobody can convince you that JBoss is fantastic - it doesn't cure cancer or send space shuttles to mars. It's only if you're in certain perspective, say you want to have reusable enterprise components, that something like JBoss becomes really interesting.
I would suggest getting a good java book and reading the tutorials at sun's website ( see links below ). I would then suggest downloading and reading Mastering Enterprise Java Beans 2nd Edition from The Server Side. If you are into web development, which is sounds like you are, I would also suggest reading Core Servlets and JavaServer Pages (JSP) by Marty Hall. Amazon has a good package deal on Core Servlets and Core Java ( see links below ).
** Remember to write alot of applications while you are reading. This will get you familiar with the Java platform and what IT can do for you. **
By the time you finish reading all of this ( several weeks ) and the jboss website, you will have a good understanding of what JBoss can do for you.
** Links **
Sun Java - java.sun.com
The Server Side - theserverside.com
Core Servlets - http://www.amazon.com/exec/obidos/ASIN/0130893404/qid%3D1020209936/ref%3Dsr%5F11%5F0%5F1/002-6929931-9613620