No matter what name we call them, ethics, religion, etc., there are no external rules. All “rules” are internal. Pay attention to how your actions make you feel in the long term. If the emotional consequences of your actions build you up and make you feel good (considering everything, like how people respond to you, etc.) then it is good for you. If the long term consequences of your behavior undermine your self esteem after all things are considered, then you have the option of modifying your behavior. If you feel something is lacking in your life, then maybe it is, but you have to figure out for yourself what it is. There is no universal right and wrong, only what makes you feel fulfilled and what makes you feel empty. That said, all humans evolved under very similar pressures, so what makes one fulfilled may end up being similar to what works for another. This is what gives the appearance of universality to rules of ethics or religions.

Ethics are generally a specific, usually written, codification of principles and rules for a particular area such as a profession. The principles and rules themselves are morality, and they are (largely informally) negotiated by members of society, based on what is understood to create and sustain the sort of society most of its members want to have (generally, a civil, stable, safe society with respect for individual rights and property). It is also based on what the understood and perceived boons and harms are to those ends.

While in theory a society could decide that, e.g., murder or rape are great ideas, in practice, humans are similar enough across societies that such things are universally understood as harms. Some people mistake this universality of accepting certain things as "evil" as evidence that morality (or at least some central core of morality) is an externally-bestowed thing-in-itself that is out there to be discovered and appropriated by society. This then begs the need for an author and enforcer of morality, generally a god or pantheon of some sort. In truth though, religion only ever appropriates societal morality and claims to be its inventor.

As to your "when" question -- if you want to live in the society that provides your morality, you always turn to morality for guidance. Although at times, the guidance is vague or conflicting, because like all other products of society, morality is a work in progress.