Depends and what are you interested in and if you prefer a particular programming language eg if you are good in say PHP I'd suggest helping the MantisBT project a bugtracker used by a lot projects. https://github.com/mantisbt. or say Python and enjoy genealogy I'd suggest going for Gramps https://github.com/gramps-project/gramps .

Maybe share more info on what you like so we suggest something you might enjoy?