May I ask whether it was possible for you to visit https://contribute.jabref.org? I allow myself to quote literally from there:
I am a programmer
I want to learn about code quality
Please head to codecov to see current code coverage and where to improve our tests. You can head to codacy to learn about one analysis of JabRef’s code quality.
I want to start with something easy
We collect small issues at https://github.com/JabRef/jabref/labels/good%20first%20issue and at https://github.com/koppor/jabref/labels/good%20first%20issue. You can read up there to find something easy to start with. Then, check our CONTRIBUTING.md for detailed information and further links.