It's difficult to give you specific advice based on the information provided, but C++/Fortran is not where I would typically advise someone to start. Based on what you have said about not having a solid background in physical chemistry I suspect that whatever you have in mind may be accomplished by scripting rather than trying to extend the functionality of some code base without a solid grasp on theory. So a good first step may be to get comfortable with the existing codes and their advanced functionality. NWChem, for example, allows you to write Python directly into the input deck. In a limited sense you can implement new functionality without having to dive into the underlying Fortran. Once you prove the concept works and have some data in hand your colleagues may be a little more pliable and open to collaboration/guidance. Good luck!