All any person must do to be forgiven of any sin is to confess their sins to Our Lord Jesus Christ then go out and live for God. Additionally, the only way to live for God is for the Holy Spirit to live in the heart of the believer.
Did Jesus say there is an UN-forgivable sin? Yes
"All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come."
To blaspheme the Holy Ghost is unforgiveable but it's impossible for a human to blaspheme the Holy Ghost because to blaspheme the Holy Ghost, the Holy Ghost must live in your heart and then u must reject Him. It's impossible to have the love of God in your heart and reject Him.
When Jesus said this He was speaking directly to lucifer and his demons. Jesus was telling Lucifer and all other demons that they will never receive forgiveness because they dwelled in heaven, saw the face of our loving God, served God, praised God, communed with the Holy Spirit on a daily bases and STILL REJECTED & BLASPHEMED GOD.
Proof that Our Lord Jesus Christ was condemning lucifer is seen in the verses leading up to this in Matthew 12:22-30 In these verses, the Pharisees accused Jesus of using demonic power to cast out demons. However, Jesus made it clear that He cast demons out using the power of the Holy Spirit and moreover, the Holy Spirit power can never be confused with demonic power because the demons have rejected the Holy Spirit.