You're mixing you cosmological arguments, but whatever.<quoted text>
...The basic premise of that is that something caused the Universe to begin to exist; this first cause must be God.
Unless you or anyone else can prove otherwise, you're simply stating your opinion, not fact.
The "first cause" argument when stated more completely is "everything that began to exist was caused to exist."
But the unspoken premise here is "everything but God began to exist".
This is a premise without foundation and creates a "begging the question" fallacy that puts "God" in the premise of the argument which is supposed to prove God. All you are actually saying is "God exist because God exist". A rather lame fallacy argument.