I gave this very brief answer recently when asked, "Why does the argument for a first cause of the universe conveniently stop with God? Where did God come from?"
An infinite regress of causes is logically impossible. That is, there could not have been an infinite number of successive events in the past because we never would have reached the events happening now.
This means, by necessity, if the world came into existence, there was a first cause beginning the world. Since nothing caused that first cause (by definition), the first cause 1) had to have begun this world by a decision of will (because the first event was not a natural result of an earlier event, and only a personal being can initiate something that's not an automatic result of a natural cause), and 2) had to be, as the first cause, a self-existent being that did not come into existence. That being is God. By definition, He does not have a cause.
If you ask, "Who created God?" you're really just asking, "Who is the real God--the true first, self-existent, personal cause?" because the one true God--the initiator of everything--does not have a cause.