This is my first serious post on this site, so bare with me if I happen to not quite match up with this sites standards due to a lack grammar or general knowledge.
Of course it is important being able to argue about scientific matters, evolution, big bang theory and other theories. Often I have to realize theists tend to not care about what science tells them but rather what they have been indoctrinated to believe over decades.
Having no real arguments many theists quickly turn a scientific conversation into a discussion about personal believe.
Since none of us is actually able to prove the non-existence of a god, a different approach would be to prove that there is no need for a god based on two things.
1. Question their character for feeling the need to follow simple rules, like not killing or showing respect to other people(which religious people hardly do anyways), only because they fear the wrath of their god. Should morals really be based on fear or rather on respect and common sense?
2. One very interesting question I picked up somewhere in the vast of the internet. If a tribesman lived his life in peace, with respect for others and followed every rule a religion has framed, will this person go to hell simply due to the fact that he never worshiped the theists god? This should also work if you replace the Tribesman with an infant which makes it even more personal for most people.
As this is obviously no Step-by-Step tutorial how to argue with someone I still hope this provides some help or maybe just a little poke to the brain to give some new ideas.
Me being a very funny guy I will end this post with a hilarious joke.
Two hydrogen atoms walk into a bar.
One orders a beer, the other one orders a soda.
I'd ask the theists why their moral standards are inconsistent, or at least different, from trillions of people through the centuries who believe that they are following the rules and laws of their particular god. Why do they think their god has such a poor record at getting a consistent message across to his followers through the centuries, between cultures and within communities? Is it his poor communication skills? lack of power? laziness? reluctance to get involved? How can they reliably tell the difference between someone who has the message right and the millions who have it wrong? How can they tell when they get it right (or wrong) themselves? What if their preferred religious mentors are in the 99 percent who must have it wrong (because there are about that many differing viewpoints, and they are often mutually exclusive)?
Then, I'd retell your hydrogen joke thus:
Two hydrogen atoms are in a bar. Not surprisingly, they have hooked up with oxygen. You can probably imagine what comes next. Of course, they end up getting pissed.