Who made God?
It's a little over-simplistic and invites the response "God did not need to be made". To understand the issue, we have to realize the question usually comes up in context with the Cosmological or Teleological arguments for the existence of God. Thus:
Everything has a cause, God is the first cause for everything in the Universe, what caused God? Nothing? If it is acceptable for God to be a causeless cause of everything, why cannot the Universe be it's own cause?
The Universe shows elements of design, this implies a designer, the complexity of the Universe means the designer must be especially great, that's God. But the same reason we argue that the Universe must have a designer means that God must have a designer as well. Any argument that shows God does not need a designer works just as well for the Universe.
We have to bear in mind, though, that talking about the Universe having a beginning in time is a bit sloppy. The reason is that the Universe does not exist in time, but time exists within the Universe. Before the Universe there was no time to have a before in ... much more of this and we'll need a drink.
In order for the likes of Stephen Hawking to talk about the beginning of the Universe, they have to use tricks in topology like proposing a kind of cosmological hypervolume for the Universe to exist "inside" of but understanding that this is just a crutch for the math: the Universe is "all that there is", by definition. So the Universe can have a "start" with respect to a time-like dimension of the hypervolume.
What all this means for us is that the Universe looks like it has a beginning from anywhere on the inside, but from the POV of the Universe itself, it is eternal, transcending time and space. Sound familiar? The Universe itself has the minimum qualities required for the creator of the Universe. From Occams Razor, we should accept the Universe as it's own causeless cause until some further evidence comes to light to suggest otherwise.
The atheist answer though is very simple: God is a fictional character whose original authorship is lost to history. They do not actually need to have a theory of creation to replace God because "I don't know" works too. This is a field of active scientific research and we have a lot of clues, wait and see.