WSDL files

Hi,
 
I am new to work with web services. I am working on a project where there are a set of WSDL files and inside the project we are trying to invoke the a method inside it. But I dont know how it works and it is throwing the error method not found. Could any one tell me how to invode the method in wsdl file using webservice and what are the requirements to do this.
Before invoking the method its trying ti create instance of the service but returns null. I am not sure how to do this.
 
Thanks in advance.
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