Time starts with the bang. Our current understanding of spacetime doesn't posit the existence of anything except heat and energy outside the big bang, or anything at all, depending on the literature you read. It's speculated that physics may have been very different in the first picosecond of space time. It's fun to speculate, but current science can't establish any real theory for a before. In fact, when you talk about any movement of time from the big bang, you're after - see my point about the laws of physics, forces taking a fraction of a second to work in the way they currently do.
A different phrasing of a question is what caused the big bang? The best answer I've gotten from this is it caused itself - that is in our frame of reference it was inevitable. We can't imagine anything outside the space time caused by the big bang. Physics and math can model the universe's first seconds to an extent, but our understanding of the universe can only be extrapolated from the three dimensions we experience and the knowledge humanity has derived from them.
So what was before the big bang? Nothing and from humanity's frame of reference there can't have been anything - your mind can't conceptualize "pre big bang. Any attempt to do so is modelling post big bang because of your limitations. The best you can get to is coming up for words for something that doesn't exist for you. Religion offers some comfort if this scares you, but if you have a scientific mind - at best one religion may be true and no religion can be proven in the same way that "pre big bang" is a phrase without meaning, something that can't be conceptualized.