Some book recommendations:
J2EE Web Services by Richard Monson-Haefel - it is slightly old, but a good place to start and most parts of it are still useful for those aspiring for SCDJWS 5.
Java Web Services Up and Running - One of the few newer books on Java SOAP web services.
RESTful Java Web Services
I recommend starting with learning XML. PLay around with it until you have a firm grasp of the different concepts. After that, SOAP and WSDL will be easier.