In this video tutorial recorded at useR! 2021, Gabriel Becker and Martin Maechler explain how members of the R community can contribute to R.
The full video (2h15) includes Gabriel discussing his experiences as an external contributor: some of the projects he has worked on and lessons learned. This is recommended for all aspiring contributors as a window into the social dynamics of contribution.
If you are short on time, the abridged video (1h30) focuses on the starting point for most contributors: helping address bugs in R.
The tutorial presentation slides are available in PDF format (the abridged version skips slides 12-45).
The Contributing to R GitHub repository provides links to background reading and exercises based on real bugs in R 3.3.2. A docker image is provided, alternatively on Windows you can install the R 3.3.2 binary.