In layman's terms, what was before the Big Bang, and how did the Big Bang start?

Science is a matter of accepting evidence or not, not belief. Belief is for things that have no evidence. There's plenty of evidence that the big bang happened (the universe is expanding and cooling, therefore it was once smaller and hotter - there's a lot more you can read about on the wikipedia article). However, we don't know what happened before it. That's outside our universe by definition, and we can't see outside our universe. We have some ideas about how it could have happened, but they aren't currently testable.