The Big Bang Theory states that the Universe began in a single "explosion," or inflation event from which all of matter-energy and space-time took form. The latest estimates pinpoint this inception moment to have occurred about 13.8 billion years ago. Your very thoughtful question presupposes that this inflation happened at a specific point in space. In fact, the Big Bang Theory states that space, itself, was created in this event. So, there was no space in which the Big Bang could have occurred. Unfortunately, this answer is aggravatingly counter intuitive, as we all envision explosions or inflations to happen relative to something else.
For instance, when one inflates a balloon in a room, the balloon membrane approaches the walls as the balloon inflates. Assuming that the balloon remains steady, the center maintains a constant position and all points within the balloon have a specific distance from it. In the Universe, we lack the convenience of previously defined boundaries.
The second part of your question also has a frustratingly unintuitive answer. As time, itself, was created in the Big Bang, there would have been "nothing" before it. No space. No time. The birth moment started the clock. As Stephen Hawking once said, "Asking what is before the Big Bang is like asking what is one degree north of the North Pole?" The word "before" is temporally relative. I suspect this answer might have proven to be of no help, whatsoever.