Before the Big Bang, a vacuum state with no space or time, physical laws would work here, but matter can neither be created nor destroyed, but energy ( plasma ) and matter are really the same thing like ice and water is. It seems that something was ALWAYS there (The uncaused cause just has to be accepted)

It is not sure Why the pure-energy started to condence matter but most astronomers think it all began about 15 billion years ago, others think between 20 and 8 billion years ago.
Anyway about 5.0 to 4.5 Billion the earth's nickel-iron core collected with gravation and magnatism pulling it together, and the outter areas formed over the next 120 and 290 million years (Thanks to a nearby supernova) So now about the Earth: