My understanding of the Big Bang theory (and I may be wrong!) is that in the beginning there was absolutely nothing. Suddenly and for no apparent reason this nothingness exploded. From this nothing matter was created and started to expand outwards creating protons neutrons and electrons which started to form together into elements and then compounds. From this explosion all matter in the universe was created. The explosion continues to expand and thus the universe is in a state of expansion. The theory is the generally accepted explanation for the creation of the universe among scientists. Although scientists have theories that account and explain the Big Bang after the first (millionth?) of a second they cannot explain how this nothingness could originally explode.
As scientists are currently unable to explain the point at which nothing could become something this is where the religion and beliefs of Bangism come in. The belief of Bangism is that in the beginning of the universe there was actually something there. Our current universe is in a state of outward expansion but at some point this expansion will stop and the universe will enter a state of contraction. It will continue to contract backwards until all matter in the universe is compressed back to the state it was at the original Big Bang. At this point it explodes as before to create a new Big Bang and a new universe will be created.
Bangists believe that this process of continual expansion and contraction is infinite and that this current universe is not the first universe created. Given that the process is infinite then at some point in the future a universe will exist in which you exist but under differing circumstances. In this life you may never win the lottery but at some time in this infinite process you will! The chances that the exact chain of events that led to your current existence will occur again is so small as to be almost impossible; however as the process is infinite you will eventually come into existence again and again under differing (or identical) circumstances to your current existence. In some of your future existences your life will be considerably better but in others it will be considerably worse.
Bangists to not believe in a deity type god and consider that God is actually everything in the universe. The question about the origin of the very first universe is answered by believing that the infinite process of universe expansion and contraction is circular in nature and therefore without beginning or end.