You don't write Robot Framework test cases in Java unless you plan to extend RF with your own keyword libraries. Robot Framework tests are usually written in text, html, TSV or reStructured format. In text format the separator is important, usually two spaces or the pipe character.
The Robot Framework User Guide is an excellent source of wisdom, but this guide is good one for a quick introduction.
Robot Framework also has a Quick Start Guide which you can download.
I suggest that you start by simply starting a browser and using it to navigate to eg. search engine home page to perform a search. See Selenium Library docs for more info.
Below is an example test case.
Search with DuckDuckGo
Open Browser https://duckduckgo.com/
Input Text search_form_input_homepage ATDD
Submit Form search_form_homepage
Page Should Contain Using Robot Framework for ATDD