Let me explain in a simple way:
Satan tried to be like God, but got striked down, and his punishment was to be thrown in Hell.
Since Satan is such a bad loser, he seduced Eve and Adam to commit the same sin: being like God.(Genesis 3:1-6)
God is a just judge, and since we commited the same transgression, we deserve the same punishment. But God is so merciful, he himself came to earth, as Jesus Christ, to die in our place(John 3:16). He didn't sin, he was just, but he got crucified. After 3 days he ressurrected. It's the innocent sacrificing himself to save the guilty.
Anyone who believes that Jesus died for all humanity then ressurrected, is saved. Your sins end crucified on the cross, and you receive eternal life, for free. You just need to pray, like: Dear Lord, I accept that I sinned and I regret it, and I would like to receive eternal life, in the name of Jesus, amen.