We know about Jesus only from the Bible, the scriptures of Christianity. There are no historical accounts of Jesus that survived from His time.
The part of the Bible that talks about Jesus is the New Testament. The New Testament contains four Gospels. These tell us some of what Jesus said and did while He was on earth and some of what happened to Him.
The Gospels do not tell us everything Jesus said and did. They only tell us some.
The word Gospel literally means “good news.” Jesus is “good news” for us.