Begin by reading around the site, where I address many of the reasons people are Christians. If you've use any of those, there's a start.
Simple stuff to get you thinking: there are many religions, and adherents to each are just as convinced they're right. Why is one lot right and another wrong? How many possible gods are there, and therefore, what are the chances you've got the right one?
To distinguish between agnosticism and atheism: if there is no particular reason to believe in any particular god, why believe that there is something at all?
This is essentially what I do when I want to make more atheists. I ask people why they believe. That's what happened to me: as it turned out, my reasons for believing sucked, and I realised the fact.