The best starting point is the GitHub issue tracker.
For existing tasks, the labels good first issue and help wanted are great for contributions. In case you want to start working on one of those, just comment in it first so no work is duplicated.
New contributions in form of pull requests always need to go in the dev (development) branch.
Maintainers organize prioritites and progress in the projects tab.
For coding style, please try to follow the guides in ComputationalRadiationPhysics/contributing for new code.