When humans sinned, the earth which was perfect became cursed.
God said to Adam (the first man who sinned against God),
"Cursed is the ground because of you;
In toil you will eat of it
All the days of your life.
Both thorns and thistles it shall grow for you;
And you will eat the plants of the field;
By the sweat of your face
You will eat bread,
Till you return to the ground,
Because from it you were taken;
For you are dust,
And to dust you shall return." (Genesis 3:17-19)
So creation became cursed, because of man's sin. Death and decay came about, starting with Adam and Eve. In the above verses, when God told Adam that he would return to dust, it meant that he would die and decay. Not only have people died and decayed, but all creation became corrupted. Plants and animals died and decayed too.
"For the creation was subjected to futility, not willingly, but because of Him who subjected it, in hope that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God." (Romans 8:20-21)
The only one who can save us from sin, curses, and eternal death is Jesus, the Son of God, whom God the Father sent.
Jesus came to earth and took our sins and wrongdoings upon Himself. The perfect Son of God died on a cross for all mankind in order to become our substitute before God. He took upon Himself our punishment for sins so that those who believe in Him can live.
Jesus suffered more than physical pain. He suffered on the cross beyond what we can understand, because He also suffered in the spirit and soul the punishment for our sins.
“As a result of the anguish of His soul,
He will see it and be satisfied;
By His knowledge the Righteous One,
My Servant, will justify the many,
For He will bear their wrongdoings.” (Isaiah 53:11)
Without what Jesus did on the cross, we would need to bear the punishment for our own sins when we die. But Jesus died for us so that those who believe in Him and repent (turn away from their sins) can have eternal life.
Three days after Jesus died, He resurrected bodily from death. He appeared to many disciples and then He ascended to heaven.
You can read about Jesus in the Bible in the book of Matthew:
https://www.bible.com/bible/111/MAT.1.NIV
People and the earth have been corrupted by sin. But through Jesus, all believers and the earth will be restored one day.