Nothing much you just need the WSDL of the webservice, and if not using Java 6 or JAX-WS then need some framework like axis, axis2, cxf.
Simplest use a eclipse, and follow my blog tutorial
http://soa2world.blogspot.com/2008/05/soap-client-eclipse-web-service.htmlonce you are able to test fine , in the same webservices explorer you will find option of generate client, which will do the job for you.