This is a hot debate topic, namely because it involves two strong opinions: that of theists and that of atheists. And then, of course, there is that of no opinion: agnosticism.
So, which is most accurate? Should you believe that there is a God? Should you deny the claim of God? Or should we not answer this question altogether?
First off, let's begin with the cause of the physical world around us, or our empirical reality. What caused it? We know that it is physical, and all things physical require a cause. An example of this is with a cup. It cannot move on its own, nor can it create itself. Something cannot be its own cause. In order for A's state to change from non-being to being, it would require something to cause that shift. It cannot cause that state change by itself existing.
Now that we've gone over the law of causality, and that the physical world requires a cause, we'll look at some theories for the cause of the physical world.
1) The Big Bang Theory
This theory hypotheses that a tightly packed group of atoms became so unstable that it exploded, sending matter all over the cosmos. This theory also states that this expansion is still occurring. Now, what is important to note in this theory, even if it is true, requires matter to already exist. This is not matter being created, it is matter being placed somewhere new via an explosion. We'd need a cause, a reason why all of these atoms were packed so closely together, and how these atoms were created in the first place. This does not answer our question.
2) The Theory of Infinite Causes
This theory claims that the universe's cause is an infinite chain of causes, i.e. there was no beginning to the physical realm. This means that time is infinite, causing time to be circular. This is clearly false. It is well understood that all substances attempt to be at the lowest energy configuration possible, and the same can be said about the physical world, the universe itself. This means that at some level, energy is finite. This cannot be possible in an infinitely existent world, because this would require everything to have happened before, because there would be an infinite amount of chances for things to happen. This means that there should be no energy, as it would be possible for it to run out.
3) The Theory of a "Unmoved Mover"
This theory claims that the universe's cause is that it was created by an "unmoved mover," i.e. a creator unaffected by cause and effect. The way that this is reasoned is that this mover exists in a non-physical realm, as if the mover existed in the physical realm, he would need the physical realm to exist before it exists, rendering himself contradictory. Instead, this mover exists in a non-physical realm which, being non-physical, does not bind to physical laws and properties, such as cause and effect. This means that the "unmoved mover" exists without a cause because it needs no cause: it does not require a cause because it exists in a realm in which cause and effect is not applicable.
I've scratched the surface of the existence of a God and its reasons to support it. If you'd like me to do a part 2, comment below.