Where in the Bible does it say that God is creating a new earth?

John, I am providing a fairly 'long answer' as you seem genuinely interested. You have tapped into a long historical theme in the Bible with your question. The first occurrence of the actual words ‘new earth’ occurs here at around 700 BC:

"Behold, I will create new heavens and a new earth. The former things will not be remembered, nor will they come to mind. (Isaiah 65:17).

This prophecy was made by Isaiah to Judah who would be fallen into captivity by the Babylonian Kingdom (roughly around 500 BC) but would again be saved, just as Moses had delivered them from Egypt. So by what might appear on face-value as a hyperbole, this was a promise to Judah that after they became captive they would also again return to Jerusalem. This return would be like a ‘new world’. At least it would be better than the life in Babylon.

However, the language goes far beyond what one might expect from such deliverance and seems ‘heavenly’ so within old Rabbinic interpretation before Christ these verses were taken to apply to the times of the future Messiah who would restore Israel. For example this does not seem like something normal on earth:

he who dies at a hundred will be thought a mere youth; he who fails to reach a hundred will be considered accursed. (Isaiah 65:20).

Here is an example of an Old Rabbi’s view from an ancient Midrash before Christ:

But a still more strange application of the verse occurs in the same Midrash (Par. 95, ed. Warsh. p. 170 a), where the opening clauses of it are quoted with this remark: Come and see all that the Holy One, blessed be His Name, has smitten in this world, He will heal in the latter days. Upon which a curious disquisition follows, to prove that every man would appear after death exactly as he had been in life, whether blind, dumb, or halting, nay, even in the same dress, as in the case of Samuel when Saul saw him - but that afterward God would heal the diseased. (Life and Times of Jesus, Alfred Edresheim, Appendix X, P1255).

So this Messianic expectation surrounding a ‘new world’ was that the Messiah would bring all Israelites back to Jerusalem and heal everybody in a sort of utopian society. This is how they would interpret the next verse:

25 The wolf and the lamb will feed together, and the lion will eat straw like the ox (Isaiah 65:25).

You can feel the heavenly impression of this peaceful image. This added to a Messianic expectation of a truly ‘new world’. Besides the life after their return from Babylon was not really so exalted, they did not even have their own independent King, but were vassals of other Kings. Therefore this only increased the Messianic expectation leading up to the time of Jesus from Bethlehem. The Jews were subject to the Romans at that time.

With this Messianic expectation that a ‘new’ world would be created, the New Testament picks up on the theme in fulfillment and in prediction of another future fulfillment. First, in the current fulfillment a ‘new’ world was already created by Christ’s death, resurrection and ‘coming in power’ by his sending the Holy Spirit in Acts 2. This was a ‘creation of a new world’, or His ‘kingdom’ and so we call it the new covenant. Those who believe in God are called members of this kingdom and are born again into it, having a ‘new life’ in this ‘new world’. This was like God ‘shaking the whole heavens and earth’, so that a new heaven and new earth would result from what ‘could not be shaken’ (See Haggai 2:6).

Second the New Testament also indicates that this ‘shaking’ is not yet fully accomplished:

10But the day of the Lord will come like a thief. The heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything in it will be laid bare. 11Since everything will be destroyed in this way, what kind of people ought you to be? You ought to live holy and godly lives 12as you look forward to the day of God and speed its coming. c That day will bring about the destruction of the heavens by fire, and the elements will melt in the heat. 13But in keeping with his promise we are looking forward to a new heaven and a new earth, the home of righteousness. (2 Peter 3:10-13)

So your answer your questions: Jesus has made a new world and is still building it, all are welcome to join by placing their faith in Him. Also there will come a time when the whole world will be destroyed by fire at the second coming of Christ to ‘judge the living and the dead’.

Regarding why God regretted that he made man after the world had become so filled with sin, it is a anthropomorphism, or personification, of God as though he was a man, in order that we can understand what would otherwise be incomprehensible. In other words God has his own reasons for making man, even knowing they would sin and fall, but as we cannot fully comprehend them, He just wanted us to understand that sinful man is far beyond what his purpose was. Yet no worries, now God has shown us how He planned to kill sin, and kill, death, it was in Christ so that:

For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. (John 3:16)

This is both, how God made a new world, and how we can enter into it today.