John was a close friend of Jesus. For three years, John lived with Jesus. John saw how Jesus behaved. And John believed that Jesus is God (John 1:1-18). John wrote his Gospel (good news) so that other people would believe this too (John 20:31).
John saw that Jesus did many wonderful things (John 21:25). Jesus cured ill people (John 4:43-54). Jesus walked on water (John 6:14-24). He provided food for 5000 men (John 6:1-15). Jesus was alive again after his own death (John 20). John describes these events in the same way as the other Gospels.
But the main part of John's Gospel is not about these wonderful deeds. Instead, many chapters contain Jesus' words. For example, John 13-17 contain Jesus' words on the night before he died. John thought that Jesus' words were wonderful (John 3:34). And we must accept Jesus' words. If we believe Jesus' words, then we will live with God in heaven always. But if we do not obey Jesus, God will punish us (John 3:36).
Jesus died so that we can live with God in heaven (John 3:16). This is why we must confess our evil deeds to God. And this is why we must invite Jesus into our lives. Then, we will become the children of God (John 1:12).