How to contribute to the developpement

Hi, I'd be happy to contribute to STK developpement.
I never contributed to an open-source program before, I never even programmed in team, so I don't know really what I have to do.
How to submit modified source code files ? (and where)
I never contributed to an open-source program before, I never even programmed in team, so I don't know really what I have to do.
How to submit modified source code files ? (and where)