Audacity is being developed on Github. Most information necessary to contributing code can be found there, such as building instructions.
Note that Audacity currently is undergoing major restructuring. As such, we can't promise a swift review or merge of any pull requests you make.
As the codebase is over 20 years old, it's not massively beginner-friendly (though this should get better once the refactor is complete). If you are new to C++, you may want to try doing something else instead first - developing a plugin might be an idea.