Which version of jboss are you setting up?
Look at the example file in JBOSSHOME/docs/examples/jca/postgres-service.xml
And read this post in the forums:
http://www.jboss.org/modules/bb/index.html?module=bb&op=viewtopic&t=forums/ the example and the instructions in the forum, I was able to get it up and running fairly quickly.
If your using 2.4.6 then the jca stuff won't help you.
Typically to set up the datasource,
Set up a datasource in jboss.jcml. There should be a default DS that you can copy/paste with your postgress details filled in.
Make sure your jdbc driver is in jboss/lib/ext
Configure your jaws.xml to point at your datasource
And then restart. The key lines of debug to look for are :
[INFO,JdbcProvider] Loaded JDBC-driver:<postgress driver>
[INFO,yourDS] XA Connection Pool yourDS bound to java:/yourDS
If you see these two guys, should be going pretty well. Let me know if you get stuck wih any steps.