What language is used to write the server part of the web service does not matter.
How to go about writing a client depends on the service itself. I guess that you have a SOAP service on the other end in which case I suggest that you look towards the javax.xml.soap package, you can find help here: Tip: Send and receive SOAP messages with SAAJ from IBM DeveloperWorks. I have previous used exactly the same document to enable me to write a SOAP client and it worked perfectly and was fairly easy to develop.
I would suggest that you learn a little about web services and XML before you have a go at the example because they dive right into the API abd expect you to know the basics.
I am also new. Recently I tried one. Download Axis2 at http://axis.apache.org/axis2/java/core/. Download the one that is aligning with the JDK you use. They have 'Quick Start' and 'Installation Guide'. Please go through them at least once. In Axis2, there is a tool called wsdl2java. This can create client code. We just need to use them to test it. You need necessary library (jars) in the environment where you develop/run this. Things like security, etc might come into the picture.
There is a lot more to Webservices than what I said. As Philip said, please read more information as you go along.
subject: Java Client to call deployed Java Webservice