I would like to think that the first cause argument is known practically by everyone in this forum. So its nothing new. This comes with a request so this is honouring that request.
The first cause argument is one of the cosmological arguments. Many have posited various arguments in history and the most prominent argument is of the philosopher Imam Ghazali. One of the significant differences between two of the philosophers in this topic, Avicenna and Ghazali is that Ghazali sticks to one single or fundamental first cause argument which has separated other cosmological arguments from his Kalam argument but Avicenna makes one Kalam argument with the contingency argument as well, and he seems to take a pragmatic school of thought.
Simply put, every originated thing has an originator, and since the world is originated, it has an originator. This would argue that if its "first cause" argument on the table, that goes into validating the first cause, and the God argument is a separate argument from the first cause argument and is not the topic at hand.
The first cause argument is simply a logical premise by premise argument.
P1: Every being that has a beginning has a cause for its beginning.
P2: The universe has a beginning.
C : Thus its "possesses" a cause for its beginning.
Why is this a valid argument that there is a first cause?
Its a logical argument that banks on logical pondering based on exactly what is concisely explained in the argument itself. To elaborate or expand on it, philosophers argue that every being is contingent, which means this being can exist in other ways, contingent upon something else, and that "something else or other being" has a beginning, and if that being is contingent, it would be contingent upon something else. This will go on forever and ends up in an infinite regression. Thus the conclusion is that the universe has a beginning. Now it has to be applied to the argument above.
This is why the first cause argument is a valid argument for a first cause. In its primitive nature this argument is not arguing for a God which carries a lot of baggage and immediately everyone goes into a top down argument. Thus God is a completely separate argument, which is addressed by the Kalam cosmological argument philosophically, it its not the scope of this thread.
Peace.