What does the following quote mean: "There is no right or wrong only thinking makes it so"?
It's a statement of what's called moral relativism. This is the idea that there is nothing that is truly right or wrong and that what we think of as right or wrong is really just determined by our society. In other words, what we (or our society) think about something is what makes it right or wrong.
There are a number of examples of things that are seen as perfectly okay in one society but that are frowned upon in another. One example would be the way women dress in our society. We think it's normal, a devout Muslim would think it's wrong. Or on the flip side, we think cock fighting is completely immoral and we ban it while in the Philippines, for example, it's perfectly legal and my uncle makes money raising fighting cocks.
So the quote just means that nothing's truly right or wrong and that right or wrong is all in the eye of the beholder.
It is a theory of living that negates the importance or function or morality. For example, if you believe that a lifestyle is okay then it is okay. If you believe that one behavior is okay then it is okay "for you". According to this theory, all typical human actions or behaviors are socially prescribed but not an expression of morality, they are not inherently correct or incorrect.
What we think is generally right or wrong, or generally accepted to be so comes from thought, as there is no real definition as to what is right or wrong. Note that it is what is "generally accepted". This shows that communities and societies do dictate and can twist what is right or wrong.
It is a theory that states that to believe in the essence of right and wrong, one simply has to think about it to make it so. For instance, if I murder somebody and think nothing of it, I could not think it is neither wrong nor right, it is just there. But, on the other hand, if i were to wonder if what i did was right or wrong, then an outcome may happen of being right or wrong.
I also wanted to show the sociological view of the right or wrong approach. In society, there are things called Norms and Transgressions. Norms are the expected behaviors of members of the society, while Transgressions are the breaking or not following of these rules or expected behaviors. A Norm is only created on the belief of right and wrong, so thus it is the societies basis and belief on what they think is right and wrong that they make their Norms.
So, if society did not believe in right and wrong, then there would be no Norms, and surely no Transgressions. Thus said, it is in societies power to dictate what is the right thing to do, and what would be not doing the right thing.