Answer:
It would depend on what time period you are talking about.
in most ancient cultures (such as Egypt, Assyria, Rome and the Greek city states) prisoners of war from defeated countries became slaves. Yet in others (like Rome or ancient Israel) you could also become a slave if you had fallen into debt. In some cases a person would voluntarily go into slavery to avoid poverty or to pay off a debt. It's complicated. Most commonly though they were born slaves, from a slave mother (normally)