There are some things we can never fully understand with our very limited human ability to know. One of these things is God himself. Seeing as there is only one God, we can't really compare him to anything else that we know of. We could say he is like this is some ways, or that he is not like this in other ways, but whatever we compare him to will be inadequate in describing who he is. That's the thing: he is the Creator, and all other things are creatures. How can the Creator be compared to his creatures without losing his awesomeness in the process of this comparrison?
One of the qualities or characteristics of God is that he is eternal...he is, always was, and always shall be. He has no origin, no beginning, and no end. We can not say "This is where/when God began" because in order to begin, there has to have been a time when God was not. In order to begin, then it would make sense too to say that there will be a time when God ends. The great mystery that is hard to understand is that he simply is. God is. God has been. God will be. God is the creator of all things, and nothing created him: he just is.
The nature of how God is (the Trinity) is almost as complicated, but that isn't what you asked about. All we can really know is that God is. As he said himself: "I AM." That is a clue itself: He is, and above all, he is all. He is existence itself and is the origin of all other things which exist. This falls into the "It will make sense when you die and come face to face with God" category. If you think about it too long, your head just might explode.