In biblical times, what did a person do that made him considered a prophet? Did he have to perform miracles?

The basic qualification was that he had to receive revelations from God. In order to prove to others that he was receiving these revelations, it was customary for people to ask him for signs—i.e., miracles that could be seen or predictions of near-future events that could be verified. If these were forthcoming, people had reason to take seriously his claim to be receiving divine revelations.