Hello there,
Good to have you on board.
Development on tge bitshares code base is decentralized. The main repository on github.com/bitshares/bitshares-core has all the issues in the issue tracker. There is a small team of trusted professionals that maintain the repo.
The usual procedure is to fork the code, make your improvement and send a pull request for review. If you do bigger changes (e.g. on protocol level) then you may need to also write a BSIP (specs) amd discuss is prior to getting your hands dirty.
Hard forks of the code require a voting through shareholders.