Slavery was accepted as normal in the ancient world
Make your enemy work for you
How to become a slave?
African slave boy
Not paying your debts
Not being able to pay one's debts could also bring one into slavehood. The debtor could try to make an agreement with his creditor to work his debt off as a slave or he could be sold outside the city. Someone could also sell himself or his children into slavery, e.g. he needed the money. In 326 bc this form of slavery is banned..
Sold by your father
A father could sell his children into slavery. This was seen as acceptable because somebody could be so poor that he couldn't feed his children properly. (There was no anti conception in those days.)
Children from slaves
Achild born out of a slave woman would become a slave, even if the father was the owner of the slave or another free man. Children born by a free(d) woman were free. There were no slave farms to breed slaves. In Rome, successful as it was in warfare, there was no need to, since slaves kept pouring in from conquered cities. This didn't mean slaves were cheap.
If a slave had desired qualities, like being literate, he or she could have a good life. But it was also possible that someone with qualities of a good fighter was made a gladiator. Romans were quite generous in freeing slaves, who had done good work for a long time (manumission). The less fortunate had to work at a farm or in a mine,
Too many slaves led to problems, like peasants being replaced by slaves
and then became unemployed and poor.
Too many ill-treated slaves led to several slave
revolts. Of these, the revolt of Spartacus is the best known. Later laws would forbid to illtreat slaves. Killing ones slave for no good reason became punishable.