The documentation is very good. https://docs.mendix.com/howto/logic-business-rules/extending-your-application-with-custom-java
However, the thing that helped me most was downloading the Community Commons module and looking at the code for the actions there. They are simple, but they cover a lot of functionality, so you can learn a lot just by following the code.
Hope this helps.
Go through the following learning path:
And Download community common from marketplace and try to implement them.
Hope it will help you,
In addition to Robert’s answer:
Java coding is in my opinion not the first step to code in Java but to know what’s Java and why should you use it, how does dependency management work, etc. Also the object oriented approach (OOP) and doing the right design. This helps you to build Java Actions in a more clean and readable way.