Coming straight from the Museum in Nîmes, France where there is an arena :

You could become a gladiator in one of several ways. Many gladiators had been taken prisoner in one of the numerous wars waged by the Romans. They gave the spectacles a multi-ethric character. Others were ordinary slaves whose owner had sold them to a gladiator manager. Some had been found guilty of a serious crime. None of these people had any rights Free men also became gladiators. These so-called auctorati elected to become gladiators of their on free will. They signed a contract relinquishing their civil rights for a given period. Mary hoped it would make them rich or bring eternal fame and glory. Among them were a number of high-placed officials, such as senators. Emperor Commodus even liked to appear as a gladiator. Gladiators lived in ludi, a cross between a training centre and barracks. Each was trained to become a certain type of gladiator. Life was tough. As a rule gladiators fought twice a year. Few fought more than twenty battles in their career. Many didn't survive beyond the age of thirty.

A gladiator was trained in the ludus by experienced former gladiators. They trained both in the morning and in the afternoon using heavy wooden weapons and a reed shield. Corporal punishment was common practice. Research into the bones of gladiator corpses reveals that they were fed a muscle-enhancing diet consisting largely of barley and beans and rich in minerals.