The beauty of web services is that it doesn't matter in which language they're implemented in - the client can use whatever language is convenient. Your favorite Java SOAP stack (whichever one that may be) probably has a tool that can generate client code from a WSDL, possibly called wsdl2java or some such.
As Lester said, you don't need to bother about where the actual webservice is running unless you have bumped into "interoperability" problems (if the webservice written is not adhering to WS Basic Profile 1.0/1.1 standards).
1. The first thing you have to do is to pick up some Webservices API framwork to generate java stubs out of WSDL for you. There are different API available (open source as well as proprietary) Axis2, XFire, IBM WSIF, JAX-WS, etc. You have to consider which web/application server (+version) as well as J2EE/JEE version you are using before picking one of these. In most of the cases Axis2 is a good choice atleast in my opinion.
2. If you use Axis2, then go through its manual to get basic ANT script to generate java stubs.
3. Once stubs are generated, you can call webservice just like any POJO class, the underlying job of (Marshalling, Serialization, Endpoint invocatio, etc.) are handled by the framework generated proxy classes.
4. If you managed to get everything working but failed in invoking webservice call successfully, there could be then interoperability issues, which can be fixed by minor changes in your WSDL, provided in the form of Java2WSDL mappings within ANT task.
5. Here is a basic ANT script, which will give you some idea on how stubs are generated from WSDL.
If you want to keep it simple, all you need to create a web service client is in JavaSE 6, including the wsimport command line tool that will help you create client artifacts from a WSDL.