First you have to decide on which projects of OSGeo interest you.
You can find a list of projects here - Projects Archive - OSGeo
Note that although all these projects are part of OSGeo, each has its own repository and website.
Are you planning to submit via Google Summer of Code or just on your own?
pgRouting for example is a C++ project that is involved in Google Summer of Code (GSOC) each year. Not all projects are and not all projects are equipped to handle students. This is not to say they wouldn’t welcome new developers, just that they are not used to having new developers.
If you are interested in pgRouting development, you’ll want to join Pgrouting developers - OSGeo Discourse and introduce yourself and your interests as detailed in About the pgrouting-dev category.
pgRouting also has an intro for GSOC students which is useful even if you want to get involved separate from GSOC. We have an ideas page and prior projects people have done listed here - Google Summer of Code · pgRouting/pgrouting Wiki · GitHub.
GRASS is another project that gets involved yearly with GSOC so is a good comforting place for new developer. To get involved with grass, join the developer group and introduce yourself as detailed in About the GRASS GIS developer category, also check out their page on contributing - Contribute
You should also check out the osgeo GSOC 2024 page which lists other projects that were involved in 2024 and some projects mentored - Google Summer of Code 2024 Ideas - OSGeo