Atheist how do you define/decide right and wrong since for you there is no sin?

I would like for you to elaborate please. The question is meant in a respectful manner. How do you begin to call something right or wrong? Does culture define right and wrong or if you do, how do you reason? Perhaps there is no right in wrong in your eyes, only opinions. If so, how do you weigh opinions yours and of others? By how closely they fit yours?By how they affect society? Or could you care less? How do you reason?