We know about Jesus' life and teachings based upon the Bible. Particularly the New Testament (Which contains the books of Matthew Mark, Luke, John, Acts, Romans, Corinthians, Corinthians, Galatians, Ephesians, Philippians, Colossians, Thessalonians, 2 Thessalonians, 1 Timothy, 2 Timothy, Titus, Philemon, Hebrews, James, 1st Peter, 2nd Peter, 1st John, 2nd John, 3rd John, Jude, and Revelation)
We also know because of eyewitness historically documented accounts from Roman, Greek, and Jewish historians who were not correlated with Jesus or his teachings.