There are various criteria which people have suggested to separate possible fact from possible fiction in what the Bible says about Jesus. Since the Bible and apocryphal gospels are the only detailed accounts of Jesus' life that exist, there's a good bit of reading between the lines and inference about what parts of those writings are myths and myth-making vs what is actually historical.

For example, since the Bible was written by early Christians, anything that might have been embarrassing to the early Church but remained in the Bible is probably true. The reasoning is that there would be no incentive to disseminate fabricated or dubious claims that would embarrass the new religion. Crucifixion was a deeply dishonorable method of execution, reserved for the worst people/crimes, so Jesus' crucifixion is generally regarded as historical due to this criterion. Furthermore, any sayings that break from Jewish tradition and/or from the teachings of the early Church might also be more likely to be authentic, since again, there would not be the impetus to write untrue or dubious sayings down if they would be deemed too controversial.

Another thing to consider is that the Gospels were written at least 50 years after Jesus' death, and probably relied on a lot of oral (rather than written) accounts. Therefore, pithier, catchier sayings are (in some scholars' minds) more likely to have survived than long winded tracts.

Through this kind of analysis with various criteria, scholars generally agree that

-Jesus was born in Nazareth

-He was a religious leader and professed healer

-He was baptized by John the Baptist

-His teaching was at least somewhat eschatological (AKA concerning the end of the world)

-He taught entirely in the areas of Galilee and Judea

-He did something at the Temple to upset both Roman and Jewish authorities

-He was crucified in Jerusalem

-He had a following of disciples who were also persecuted, but continued his teaching

The state of scholarship being what it is, you can find people who disagree with some or all of these points, but I think the vast majority would agree on at least these biographical details of Jesus' life.

This is a vexed question, to some extent because there is a a lot of disagreement about specifics and there aren't very many things scholars widely agree on. Basically they agree that Jesus was a Jewish religious leader from Nazareth, who was baptized by John the Baptist and who was crucified in Jerusalem during a Passover week. Some other things that are mostly agreed on are that he was a healer, that preached the coming of a "Kingdom of God," and that he was probably crucified following some sort of disruptive event at the Temple and that he was formally charged with claiming to be the King of the Jews. Opinions start to diverge after that as to what he said or did or taught, but the points above are the ones thought to be the most certain.