A theist is someone who believes that one or more gods actually exist.
An atheist does not have a belief in a god, not necessarily that a god or gods don't exist, just that there is not enough evidence to warrant a belief that a particular god does exist.
The burden of proof is on the theist, or a god, to prove that a god exists, not on an atheist to prove that a god doesn't exist.
As to the question of morality, atheism is not a world view or a system of morality. It is simply a rejection of the claim that a god has been shown to exist.
Therefore, your question of atheism and morality is highly biased and leading, in other words, preaching to the choir. Those who do not believe in a god, and therefore, don't believe the teachings of religion based on faith are free use other means of evaluating morality across the strata of human experience, culture, science, philosophy, and logic for the common good and well-being of society.
As a corollary, those who do not believe in the divine nature of religious scripture, say the Christian Bible, are free to read it objectively for what it is, an incredibly immoral, contradictory, and absurd collection of archaic stories that illustrate why atheism is the superior position.
If you'd like to see the ignorance and dishonesty of people trying to defend the morality of their religious beliefs, may I suggest checking out the YouTube channel - The Atheist Experience. The Atheist Experience is a call in show, usually hosted by Matt Dillahunty, where theists try to argue for the existence of a god, and generally prove themselves to be in denial of and ignorant of the immorality of their own beliefs.
The show also does an incredible job of explaining where atheists get their morals from, which is the question you originally asked, but something tells me that you weren't really looking for an answer!