Heaven, as in the place where God lives, did not have a beginning, it always was because God had no beginning. Fallen sinful mankind could not enter into heaven until Jesus died on the cross, that allowed humans, redeemed souls, entrance to heaven.

Before Jesus died on the cross and paved the way for us to enter into heaven, the souls of the dead went to the realm of the dead, Hades. Hades has 2 sections, paradise and torment and those sections were divided by a huge burning gulf that nobody can cross. This is explained by Jesus in the parable of the rich man and Lazarus. The righteous people went to paradise and the unrighteous went to the place of torment awaiting judgement day.