The Greek word for "after" is "meta". Once he finished writing Physics, Aristotle wrote Metaphysics, which literally means "the topic that comes after physics". It's not the most creative title ever, but then again, he probably never expected his lecture notes to be published.
Metaphysics deals with being and the nature of existence. Aristotle notes that there are two kinds of being: necessary being and contingent being. In other words, there are some things which have to exist, and some things which do not have to exist. For example, when I drop an apple, it necessarily falls to the ground. It can't not fall to the ground. Compare this to what happens when I tell a joke. The audience's laughter does not necessarily happen; it is possible they will not laugh.
Aristotle's first major point is that it is impossible for everything to be contingent. There must be at least one necessary thing. And this is because of the way contingent things exist. A contingent thing does not exist until it is generated, at which point it starts to corrupt until it ceases to exist. If a thing exists contingently, that means that, in a previous time, it did not exist. If everything is contingent, then there was a previous time when nothing existed. Or, to put it another way, if everything could possibly not exist, it is possible there was a time when nothing existed.
To summarize, if you claim that nothing has to exist, then you get yourself stuck with a situation where nothing existed at all. And then nothing would ever begin to exist, because there would be nothing to make the non-existent things exist. Therefore, it is impossible for everything to be contingent; there must be at least one thing which exists necessarily.
So, there must be at least one necessary thing, to explain the existence of non-necessary things. And as Aristotle's other proofs have shown, we cannot have an infinite chain of necessary beings, getting their necessity from other necessary beings. The chain must begin with something whose existence is completely necessary, in and of itself. And this is what we call God, whose essence is existence.
To put it another way, God causes his own existence. This is your answer to "Who created God?". Since God's existence is necessary, God is uncreated.
There is debate on whether or not multiple necessary things exist. Aristotle thought they did. St. Thomas Aquinas agrees; he says that souls and angels have necessary being, because they are immaterial and non-composite. But even if they have necessary being like God, they are not God, seeing as they get their existence from God.
That is Aristotle's final proof for the existence of God. Tomorrow, I'll do another Q&A session, then skip ahead a thousand years, to when the next major proof for the existence of God was formulated.