Well, in this world of cause and effect we believe that there has to be a beginning of something and then there is an eventual end.
So can we define when God began? No we can't. Can we determine when God ends? No we cannot do that either.
Now we can see neither does he end nor begin like creations do, so we have to conclude that he is beyond the way we understand things in this world.
Muslims believe that everything was created by God and he is uncreated. Its as simple as that but I wanted to give you some logic before answering it might make more sense.
God is beyond all the combined imaginations of mankind, even greater than that.
Beyond all understanding, Muslims do not limit God to one thing, he is without beginning or end. Any description of man to God is always limited and doomed to failure.
As a Muslim I would explain it like this logically;
We cannot say when God began, neither can we say when he finishes. So he must be uncreated, for every created thing has a beginning and an end. So we are unable to define this so we must come to the conclusion that God is uncreated.
One of main problems that people have is due to them trying to understand God using baseless premises. What I mean is that man tries to understand God, from a human perspective using this world as a reference. The Laws of creation apply to creation not to what made the laws.
Does this make sense? Please contact me if I haven't explained this correctly.