Things in the universe exist in time. They come into being at a point in time, and we explain them, at least in science, by giving some initial state that is their cause, and some law of nature that things like that are always produced by causes like that.
Once we have given the cause and the law of nature, things are explained - we know why they are the sort of things they are. So turtles are caused by other turtles, in accordance with laws of nature that tell us how mummy and daddy turtles produce baby turtles.
Everything that is in time is explained, it seems, by reference to a law and a preceding cause that brings it into being. But if we can think of something being in time, then it follows by simple negation, that we can also think of something not being in time. Call that an eternal thing. If there is an eternal thing, then it could not be brought into being by anything else, since it could not be brought into being at all.
Since it is not in time, there never was a time when it was not, and it could not possibly be brought into being.
Perhaps an eternal thing could be dependent upon a more basic eternal thing, though that dependence would have to be eternal too. Presumably, however - and certainly, if there is to be a final explanation - there is at least one eternal thing that does not depend on anything else for its existence.
Such an eternal thing has no possible cause. It is either is or it is not. But eternal things could be the cause of things in time, and indeed in modern physics time is often said to be caused by a super-temporal reality beyond it (for instance, by the vacuum state posited by some quantum theories).
Notice that a timeless cause is not the first thing in time. It is the cause of everything in time, whether time has a beginning and an end or not. From the timeless all times arise, and all things in time may depend upon the timeless - but they do not come after it.
Could there, then, be a timeless turtle that is the cause of all temporal turtles, but that cannot itself be caused or be brought into being? The trouble with timeless turtles is that turtles seem, almost by definition, to be things that come into existence, grow and then die. So it is really hard to think of a timeless turtle. Even an immortal turtle, that lived forever, would not quite do, because it could in principle die at any time. What we need is a truly timeless turtle, which is not in time at all, and so could not possibly die, or be born either.
If we cannot think of a timeless turtle, perhaps we can at least think of a timeless cause of the whole pile of turtles. And that, really is the first part of the answer to question, "Who made God?" The answer is that God is eternal, so nothing could possibly make God, cause God, bring God into being, or kill God either. Once you see that God is eternal, you will never again ask the question, "Who or what made God?" You will see that the question does not make sense.