Just study the Web service fundamentals for both WS-* and RESTful. Understand the pros and cons of each.Read about WSDL, XSD, XQuery, XPath, XML, JAXB, etc.
Learn about the tools to use for Web Service development like
SOAP UI, Firefox "poster" plugin, etc.
If you have not used Web Services before, go through some tutorials.
No one can guess what questions get asked.