Where does Jesus say he is going to prepare a place for us?
The short answer
In John 14:3: "And if I go and prepare a place for you, I will come back again and take you to myself." For the question of going to heaven at death, Luke 23:43 is more to the point: "today you will be with me in Paradise."
The passage is John 14:3: "And if I go and prepare a place for you, I will come back again and take you to myself." That verse may not settle the question at hand.
Luke 23:43 hits it squarely: "today you will be with me in Paradise." What is at issue is the particular judgment at death rather than the general judgment.