As the sreehari explained, first you need to have JAX-RPC API to include in your project. Once you have done that, the next thing comes, How to call web service.
Web Service can be created in any language like Java ( servlets ), PHP or .net, I have worked with all these three. for Java & PHP you can directly call the web-service, while for .net you need to create stubs.
Steps to create stubs. Once you create these stubs, you can simply call it like you call other methods in code and pass the argument ( if necessary ).
Also look at this quetsion How to use web service in J2ME application ?