I shall not sacrifice myself or others
The Objectivist Ethics was a paper delivered by Ayn Rand to the University of Wisconsin on Febuary 9, 1961. It was later published in Ayn's book The Virtue of Selfishness. In it Ayn details the Objectivist view of ethics and how to live a moral life. It goes something like the below.
As a sentient being capable of higher reasoning, man's ultimate means of survival is his ability to think rationally and apply his reasoning to sort out problems. Other animals have their instincts (like the beaver automatically constructing the dam, or a fox hunting for food), but man must employ his reasoning to go about these tasks.
Man's life should be viewed as an end in itself, as in day-to-day circumstances he must preserve it. The pleasure-pain mechanism in man is innate, part of the type of being he is. Things which maintain man's life are pleasurable, things which act against it are painful. Man can survive for a short amount of time on a self-destructive path, but eventually he will have to change his ways or perish. It is each man's moral imperative to survive.
The three values by which man can fully realize his life, says Ayn, are self-esteem, reason and purpose. Self-esteem is the ultimate key to happiness, and happiness is the state in which man may fully enjoy and take most value from his life. Purpose means man engages in productive work, from which self-esteem will spring. Reason is the over-riding factor, the thing that must govern man's decisions.
For people who hold ethics to be in the realm of the mystical, their ethical imperatives come from "above", and it is not a precondition of that which comes from "above" that it make sense. If man lives his life fully employing his reason, he will be an independent, honest, just and rational man. This is the state in best to live one's life, because if you begin to let your reality be invented by someone else (a priest or a peer group) you have already begun to sacrifice yourself to the whims of others against your will. Man should never give in to the whims of others or his own whims of the moment.
One final note, the above is not an excuse for hedonism. Happiness should be the purpose of moral values, but not the standard - ie., something isn't good just because it makes you happy, but rather the code of ethics should be geared towards giving man the means to achieve happiness. Men's whims and desires have no place in an ethical code.
Nor does it the above mean that man has the right to sacrifice others for himself, through theft, fraud or any other crime. In fact, man has no right to infringe on the rights of other men, the only just relationship for men is that of traders, exchanging things of value with one another for mutual benefit.