Entire university and college courses are designed around this topic. I don't know how much you are going to get out of a reddit comment, but here are a very few key figures/ideas.

Socrates did not write anything himself, but shows up in the writings of his students, mostly Plato. The figure Socrates was concerned with the area of Virtue Ethics, which basically means that a man will pursue his pinnacle, and through this will do what is right. To act immorally is an act of ignorance, and ignorance is bad.

Aside from Virtue Ethics is the branch of ethics termed Hedonism. The ethical man will maximize pleasure and minimize pain. Epicurus presented the idea that pleasure, once properly understood, will align with virtue.

Aside Hedonism came the branch of philosophy of consequentialism, and Jeremy Bentham and John Stuart Mill pushed the idea of Utilitarianism, which is Hedonism applied to consequences. Actions have a measure of utility, which is the measure of pleasure, well-being, and general lack of suffering. The ethical man will maximise utility, even if some people are harmed in the process.

There is another branch of ethics called Deontology, which examines acts and not consequences. An act can be morally right regardless of the consequences. Immanuel Kant argued that the moral man will act from duty, the categorical imperative is the proposition that certain actions are necessary and must be obeyed. "Act only according to that maxim whereby you can, at the same time, will that it should become a universal law" is a different way of saying "do unto others as you would have done unto you". Kant was more interested in the ideas of the categorical imperative and how it applies to perfect duty. There is a moral obligation to act by maxims that do not result in logical contradictions when we try to universalize them. If it were universally acceptable to lie, then all truths would be assumed to be lies. You would not lend money, because you would assume that the borrower would not pay you back.

That is just a small bit of the history of the philosophy of ethics, which brings you from ~400 BCE to the 1800s. The nature of reddit means I have glossed over huge swaths of history, and probably ignored or forgotten some very important people and points.