Well, the first reason that I always give is that there is insufficient evidence for the belief in any supernatural being, let alone a god (and let alone the specific god that your religion believes in). I always preface an explanation of my atheism by noting that I do not believe in the Hindu gods, the Roman gods, the Celtic gods, the Zoroastrian gods, the Christian god, Cthulhu, the Norse gods, leprechauns, pixies, or any other supernatural creatures. My reason for not believing in all of them is identical: no one has ever produced any evidence for the existence of any those things.
The burden of proof is always upon the person making the claim that something exists. If I claimed that a leprechaun lives in my backyard, it would obviously be up to me to prove to you that it exists before I could expect you to believe in it. It would be quite silly for me to insist that you believe that it exists unless you could prove otherwise. Now imagine if you inspected my backyard and found no sign of a leprechaun, and I said, "Well, the leprechaun is intangible and invisible. You can see him through events that happen in the backyard, like acorns falling off trees, holes appearing in the lawn, and rainbows appearing in the sky over that big tree...." Not exactly convincing evidence for the existence of a leprechaun, is it?
The arguments for a god seem to take two general forms:logical arguments for the necessity of such a being and emotional appeals to personal experience.
Obviously, emotional appeals to personal experience can't demonstrate that a god exists because people of all faiths -- including conflicting faiths -- all have personal "experiences" of their gods, and all of those faiths can't be correct. What that demonstrates is that it's possible for you to fool yourself into having an experience and mistaking your own emotional energy for something outside of you.
The logical arguments are sometimes tough to get one's head around, but they often take the form of an argument from ignorance. We don't know how the universe got here; ergo, it had to be a god. We don't know why the laws of the universe are the way they are; ergo, it had to be a god. We don't know how life arose; ergo, it had to be a god. I mean, the arguments aren't phrased like that, but that's what they pretty much add up to.
And obviously, if the burden of proof is on you, it doesn't help your case to say, "Well, no one knows, so therefore I'm right."
The atheist answer to a lot of the big questions is "I don't know," which -- until we know more -- is the honest and correct answer.
That's pretty much the broad strokes. If you want me to elaborate on any particular point or address a particular argument, let me know.