Thanks for the clarification. It wasn’t bad English, I just wanted to be sure about the nature of the question.
And honestly, it was kind of a leading question – learning to use Blender and write its Python API are both good ways of learning and understanding Blender. Very likely, there’s a specific feature or workflow in Blender that excites you, or some feature that isn’t in Blender that you want to add. Maybe you don’t even know this yet! You’ll find something that excites you if you spend some time learning this basic stuff.
The fist thing to do, in any case, is build Blender.
Once you decide on a first project, start looking around the module page on developer.blender.org and read the commits and the discussions (don’t add discussion unless it is appropriate; some tasks are only for code discussion by module members). You can contact some of the developers on blender.chat . And of course, there’s no substitute for reading the code, and trying things out the old-fashioned way: trial and error.