When one gives thought to God existing or not inevitably one uses that part of their religion that tells them there is a god. However, with simple logic and research it is possible to conclude that there is a being outside of our natural existence that set everything into motion. We refer to this being as God because it is the accepted term, so I will just use God. But, for the record, I don't believe in any particular religion(because ALL religions are manmade), so God is really an inadequate term.
As someone mentioned, there is a God because of our existence. Life in it self is evidence that there is a God. Atheists will argue that life is only evidence that there is life. The problem with that is the it would negate one of the strongest laws of the universe; CAUSE AND EFFECT. There is an attempt by every religion, culture, society to define who and what God is. Here is the perpetual problem. How can we define something we can't comprehend? And atheists will say, since we can't define him ACURATELY, there must not be a God at all. Again they fall short with their logic.
Sometimes the only evidence that their was a cause is the existence of the effect. Because we don't know what the cause was does not mean there was no cause. Simularly, sometimes the only evidence of a person (or being) is the existence of that which they created. Example: Who invented the wheel? Although there have been many attempts to find this out and conflicting answers, there is one thing that is clear. We have a wheel therefore someone invented it. Just because we don't know who did it, doesn't mean it has no inventor. But, we don't make up a person and give him genius qualities either.
The same is true of God. We live. There is a natural order of things. There are physical laws of the universal. The alternative to there being a God would be that this stuff happened by chance/ at random. I think most rational people can agree that is ridiculous. Have you ever rolled dice? Have you ever rolled 10 doubles in a row? Well consider this...in order for life to have been gernerated at random it would mean that nature somehow threw 2 six sided dice and they landed on double sixes over 11 MILLION times IN A ROW! I think God is more logical than that. Don't you?