Typically, you follow project guidelines for such a thing. Since SF is the main repository apparently, I'd start looking there for a guide to contribute to the project. Alternatively, you could try to find some way to contact the development team, for example mailing list, a forum, or a chat-room somewhere.
Check if they have issues that need work, whether there is active development, and possibly some kind of queue of code patches that are "ready", ie waiting for review or acceptance by the team.
Other stuff to look for are coding style and documentation style guides, but generally, the guidelines point at them.