In your application, how are you accessing the database? Directly via JNDI or are you using a data source defined within Tomcat? If the former, you don't have to make any changes (well, you do need to copy the JDBC driver jar file to the server/default/lib directory).
If the later, you need to create a *-ds.xml file to define the data source, and copy it to the server/default/deploy directory (and put the JDBC driver jar file in the server/default/lib directory). Several example *-ds.xml files, for various databases, can be found at ./docs/examples/jca. And the documentation is at http://docs.jboss.com/jbossas/guides/j2eeguide/r2/en/html_single/#ch7.jdbc.sect
I have 2 databases in my working project in Tomcat.
One a sql that i already have working in JBoss, and the other one is the editable user database that Tomcat uses by default. It's just a xml file, that i use to login users.
I can't get the last one to work! I tried putting it in the *-ds.xml file, in the login-config.xml file...
I really, really need some help.
JBoss supports a variety of login modules, not sure if one of those will do what you want (read your xml file as is), but you can read about them at http://docs.jboss.com/jbossas/guides/j2eeguide/r2/en/html_single/#d0e17193