Follow the guide on https://manual.os-js.org/development/ for how to work with a development environment and a list of repositories to work with.
If you want to contribute, but not sure what to do, here's a list of things that I always need help with:
- Translations
- Bug-hunting
- Finding security problems
- Themes and general graphics
- Website design (like the homepage and manuals)
- Documentation
If you need help, or just up for some general discussion, head into the community forums or community chat.
You can also donate or become a patreon, which helps out covering server costs and potentially make it possible to put out bounties:
If you are a member of the official OS.js developer Team: