Paul wrote this long letter to the Christians who lived in the city of Corinth. He had set up the church in Corinth when he himself lived there for 18 months (Acts 18). After he left the city, evil things happened in the church. So Paul wrote this letter to the Christians in Corinth, to help them.
Corinth was a city in the country of Greece. The people in Greece were proud of their clever minds. But Paul explained that God considers human wisdom to be foolish. So, Christians must recognise that Jesus Christ is their Lord. He has God's wisdom; and Christians must obey him always (1 Corinthians chapters 1-2).
Then they will be able to live holy lives. They will not quarrel with each other. They will be loyal to their husbands or wives. They will not try to understand the Bible by their own clever ideas. Instead, they will be true servants of Christ (1 Corinthians chapters 5-6).
Paul wanted to teach the Christians in Corinth how to praise God together. He taught them that they should share bread and wine to remember Jesus' death (1 Corinthians 11). God's Holy Spirit would help them to work together (1 Corinthians chapters 12 and 14). And God's greatest gift is love (1 Corinthians 13).
Paul also taught about life after death. After Jesus died, God caused Jesus to live again. Many people saw that Jesus was alive again (1 Corinthians 15:1-8). In the same way, God will cause us to live again after our deaths. God is even greater than death (1 Corinthians 15).