I'm afraid 'easily' probably won't be possible, but we'll do our best. This is a question about who God is and what He's like.
The first thing you need to know is that Christians believe God is SO much bigger than us that we will never be able to 100% understand him. Even the cleverest theologians (people who study God) struggle to get their heads around the Holy Trinity, so don't worry if you still feel a bit confused. You're not alone.
Basically, Christians believe that there is ONE God, but that God is THREE persons - the Father, the Son and the Holy Spirit. This is the difficult bit. How can someone be one, but also three? A couple of helpful things to think about are:
Water, ice and steam. Are they the same or different? Well, they're the same in essence - they are all H2O. But they work very differently.
What's 1x1x1? Still 1.
See the shape to the right of your question. This is often used to symbolise the Trinity. It looks like three interlocking shapes, but when you look closely you can see it is one completely connected shape.
So Christians believe that the Father, the Son (Jesus) and the Holy Spirit are all God, yet different persons that work differently. All of them are mentioned separately in the Bible, but they are in constant relationship with one another. Jesus says "If you have seen me you have seen the Father" (John 14: 7).
This is why the Trinity is so important to Christians: it shows that relationship and community are part of God's very nature, which (a Christian would suggest) is why these things are so important to us humans.
The short answer is 'no-one' as Christians believe God always existed and wasn't made. This may sound rather illogical and mind-boggling but hopefully this illustration may help.
Suppose you see a huge domino display (search Youtube for domino tricks and there will be loads of examples). What knocked over the last domino? The most logical answer is the second to last domino. But you could then ask "What knocked over the second to last domino?" and the answer would be the third to last domino, and so on. Eventually you get to the point of saying that the first domino started the chain reaction. But what caused that to fall? Maybe it was pushed by a human. So what caused him/her to push it, or what even caused that human to exist in the first place? Eventually, if we keep asking "What caused that to happen?", we could logically get to the conclusion that the last domino fell because the universe was created.
But what caused the universe to be created? If it was God, as your question suggests, what caused or created God? And then who or what created the thing that created God?
Hopefully you can see from this example that this line of questions is never ending. I believe there are 2 options.
1. At some point there was absolutely nothing - no matter, no energy, nothing at all - and it turned into something without any outside interference
2. Something always existed
I personally find option 1 illogical and impossible. I can't imagine that absolutely nothing would just turn into something without there being an outside influence or some form of energy. I'm therefore led to the only other option: that something always existed. It wasn't created. It just was there all the time, even before time itself. For me, I believe that thing that always existed was God.
Infinitely old! Christians believe that God has always existed (see the question about who created God). He is eternal. He wasn't created and he didn't just suddenly appear. We believe he created the universe and that includes creating time, which we use to measure how old something is.