I think you can check this page: Home · SonarSource/sonar-java Wiki · GitHub
Just note, that sonar-java is a complex project and it requires some time to get familiar with it. Good news is that you don’t need to understand every single class completely before implementing your first rule. SO I suggest you read attentively this documentation and follow the described steps.
And once you start working on it, you will collect some questions and you can come back to this forum with more specific questions.
Good luck with that!
Best,
Margarita