Who was the first king of israel who united the 12 tribes of israel?

The person credited with doing that is King David; the reason the Hebrew people became known as "Jews," has to do with the fact that David was from the tribe of Judah, and since the king of Israel, was of the tribe of Judah, the name was given to the people. Going back to the ancient Word it is derived from, "Jew" literally means "of Judah." However after David's kingdom fell apart, the people became tribal again.