With every action we do, do the least amount of harm. I still assign a weight of 2 for myself and 1 for people I know and a weight of .25 for people I don't know. It seams to work out fairly well. Another rule of thumb "If you don't want everyone else to know what you have done, don't do it." This will save you much embarrassment.
Your question is so broad, an answer would require a shelf of books.
I started by reading Nietzsche's "The gay Science", "Beyond Good and Evil" and "Thus Spoke Zarathustra".
Being true to yourself. Being kind to others. Being a good steward of the Earth and all it's awesome creatures. So much more, but these came to mind first.
Booklover answers the way I would respond to this question. I think it's important to help others and show kindness.
It depends whether you are looking for a creed to follow, or you want to judge for yourself what is right/wrong.
If you want a creed then try one of the Humanist manifestos, or Atheism+.
If you are interested in philosophy/psychology then the first question to ask is: what is goodness? How can we tell what is good from what is bad? Technically, this topic is known as meta-ethics.
This is the place to start, because if you don't have a solid foundation then your philosophy will be confused and flounder.
To me, there is only one source of right and wrong for atheists, and that is our emotions/desires/feelings. We use reason and science in pursuit of our desires. (Atheists are often confused about this, by the way).
Sam Harris comes close to this, but he calls it "well-being" instead.
So, a good life is about maximising happiness. It's that simple. Who's happiness? You'll have to figure that out, but obviously it begins with yourself.
We have a bunch of desires, and life is about balancing those competing desires.
So you have to figure out what desires are important for you. Get to know yourself.
Like the rest of the animal kingdom, we are driven by desires. Just try not to be primitive about it.
Charles Darwin: "A man who has no ... belief in the existence of a personal God ... can have for his rule of life, as far as I can see, only to follow those impulses and instincts which are the strongest or which seem to him the best ones. A dog acts in this manner, but he does so blindly. A man, on the other hand, looks forwards and backwards, and compares his various feelings, desires and recollections. He then finds, in accordance with the verdict of all the wisest men that the highest satisfaction is derived from following certain impulses, namely the social instincts."
BTW, I would not start with Nietzsche. He is very hard to understand.
I do not know your particular religious belief or lack thereof, but I do note the ellipsis in the Darwin quote.
Certain apologists tend to use this quote (often dropping the ellipsis) to assert that Darwin was religious.
The entire first line of the quote, which when the ellipsis are filled has a much different meaning, goes like this:
A man who has no assured and ever present belief in the existence of a personal God or of a future existence with retribution and reward, can have for his rule of life, as far as I can see, only to follow those impulses and instincts which are the strongest or which seem to him the best ones.
As for the question of "How can we lead a good life?" The question is poorly formulated. If you mean "good" in the sense of "satisfactory," then whatever satisfies you. If you mean "good" in the sense of "commonweal," then whatever promotes the good of the most to the detriment of the least including yourself and not stepping on their basic rights as defined by society.
The dots/ellipsis are for brevity. I am an atheist. Relax. I don't see how you could interpret this Darwin quote, with or without ellipsis, as being religious. It means the same with or without the ellipsis.
Here is another quote from Darwin's autobiography:
... sentinent beings have been developed in such a manner through natural selection, that pleasurable sensations serve as their habitual guides.
While Darwin does admit to agnosticism in this section, nonetheless, in practice "pleasurable sensations" are his guide for living. He identifies the social instincts as giving "the highest pleasure on this earth". So, he is a humanist (in the broad sense of the word), and pleasure and pain are his source of right and wrong.
Watch the movie "The Peaceful Warrior".
Robert Green Ingersoll, the well-known American atheist of the nineteenth century wrote:
Happiness is the only good.
The time to be happy is now.
The place to be happy is here.
The way to be happy is to make others so.
Great quote by Robert Green Ingersoll, Dr. Allen! I SO agree! Thanks for posting that!