(This was originally posted in the comments over on a thread at Former Conservative in response to someone who keeps trying to see how things like morality, meaning, etc. can be derived from atheism, or whether such concepts are philosophically incompatible with atheism.)
Atheism 101: How Atheists Actually Think
Step One: I genuinely don't see any reason to believe this.
Step Two: Therefore I don't believe it.
Everything else - meaning of life, approach to charity, specific points of contention with Christians/Christianity/Islam/Wicca/Whatever, personal morality, fashion sense, etc. - is secondary to this. Ask any three atheists about any or all of these topics, and you're likely to get very different answers. But ask an atheist why he or she doesn't believe, and you're extremely likely to hear some variation of, "Where's the evidence?" (Some variations are less polite than others, naturally.)
Granted, that's a bit of an oversimplification, and I'm prepared to apologize to any of my fellow unbelievers who feel that I'm misrepresenting them. But if you're going to talk about what/how atheists think, I'd say that's a good place to start.