How to implement a test plan to validate the functionality of a web service using java?

The initial task will be to validate the web service using user name/password where the endpoint is {/login}. Then, to create users and retrieve their information's to validate that creation. There is so many resources in the internet and I get slightly confused. How can I get started with this task in Eclipse and do I need additional tools for the purpose ? I have only little experiences of writing unit tests using JUnit.