Though i really shouldn't be surprised by it, i still am a bit surprised that this question that somehow perplexes the minds of both the young and the old (though, i would say, mostly the young) is still going around being asked. I am hoping that some day they will realize that asking such a question is itself illogical—aside from the fact that God Himself is uncreated. Perhaps that day is today.
God: the only, true Creator
What is a real Creator? What is to create? To create is to cause something to exist that did not previously exist. While humans can come up with ideas, they cannot create things. You might argue that people bring things into existence that didn't previously exist all the time. However, that is a common misconception of what really is.
When a human "creates" something, they use things that are already in existence to do so (be it wood, metal or what-have-you). In the case of God, God does not use anything that is already in existence. Therefore God is a true Creator, and "Creator" does not imply to be created. If "Creator" does not imply to be created, what reason does one have to ask, "Who created God?"?
What about matter?
To say, "If God created everything, then who created God?" introduces a relationship between God and matter (through implication) that presupposes that if of matter, then of God also. This is fallacious, for the Creator, obviously, does not share the same limitations or properties of the very things He creates. The only relationship God has with His creation is that He is their Creator.
On everything
Some people are also confused about the word "everything," perhaps with good reason. For what does everything mean but everything (which would include God Himself—but that is illogical)? For that reason, "God created everything" should be rephrased.—but how can one get the point across any quicker than "God created everything"? So just assume that "everything" in "God created everything" means "the heavens and the earth and what He made to inhabit them."
However, there have been instances where people assume more than that. For example, i have seen cases where people have included evil to be part of "everything." In other words, they asserted, "If God created everything, then He created evil also." But that is to assume that God is responsible for the actions of His creatures. That is like saying that the person who built a baseball bat to play baseball is responsible for the actions of someone who uses the baseball bat to bully others.