on case of unity:
You must be registered to see the links
usually the bennigers tutorials you can find anywere actually provide all the art/music stuff for you only focus on code.
follow the guide, when succes, try do it again without guide, then with the little real experience and urdenstanding of game bussines adquired (because game programming is not fancy like the web/app programing) search open proyects for try mod it.
because unfortunally it is not someting you can learn easily only reading code. (well i really dont know if in case of rempy can be much more ligth to do)
I insist on tutorials because it is not something that can be explained in 1 hour. if this still seems too much to you, then you will have little chance that someone will allow you to collaborate on a project.
the other way is like raverger says, start making mods from other games, like "life play"(is hard because all infor is not avallaible on the docs/wiki and is limited to the random events), or skyrim on loverLab forums