Since we don't have a 3.0 getting started guide, I am assuming that this is a more general JBoss question. In the future, I'd recommend looking to the appropriate group for these types of questions.
If the problem is not knowing how to configure a datasource, the short answer is to copy the appropriate service.xml file from docs/examples/jca (oracle-service.xml, for oracle) into your deploy directory. Set all the parameters correctly for your database and set the JndiName to be the same as what you are trying to access from your application.
If you just can't find your datasource, I recommend using JNDIView to verify that your datasource is bound to the right place and that your application is linking to it correctly.