God is the UNCAUSED CAUSE. With a little logic and common sense thinking, you are asking a question that logically has only one answer. God is uncaused. Someone that is eternal does not have a beginning, therefore has no cause because He is eternal.
To answer your question from your limited perspective would lead you into an infinite series of causes and effects. This is called "INFINITE REGRESS". It's like asking who caused God and then asking who caused that God and on and on and on infinitely. The problem with your question is that you falsely assume that God had a beginning but logically someone that is eternal has ALWAYS existed and has no beginning therefore is UNCAUSED.
Now the evidence from science demonstrates the universe DID HAVE A BEGINNING, so my question remains to you WHO WAS THE FIRST CAUSE?
EXHIBIT "B"
THE EXPANDING UNIVERSE
Picture in your mind a balloon being blown up. Robert Jastrow the founder of NASA's Goddard Institute for Space Studies served 20 years as it's director is a physicist, an astronomer, and a cosmologist who btw claims to be an agnostic. He has acknowldeged that the evidence from astronomers demonstrates CONCLUSIVELY that the world began SUDDENLY IN A MOMENT OF CREATION. He says they found while tracing back to an earlier time the galaxies in the universe were MUCH CLOSER together than they are now. If they go back FAR ENOUGH IN TIME they've discovered that these galaxies were actually PACKED CLOSELY TOGETHER. This scientific finding is evidence giving support to the BIG BANG. Thinking again of the balloon, once it starts being blown up it expands, and relating this to the universe that has galaxies which are close together at the beginning they get further apart as it expands. This is REAL SCIENCE Kia. Science with facts that remain and don't crumble because of limitations. You can read the whole story hear.