What is a priest?
Priest blessing people who have the plague
Very soon after the crucifixion of Jesus, by around 40 or 50 AD, the early Christians began to choose leaders from among themselves and call them presbyters, or "elders" in Greek, which is where we get our word priest from. These early priests were chosen by election of their congregations - the people who went to their church. Some early priests were apparently women, because in 494 AD Pope Gelasius complained,"that women are encouraged to officiate at the sacred altars, and to take part in all matters imputed to the offices of the male sex, to which they do not belong." (Epistle 14: 26). But mostly only men could become priests.
By the 200s AD, the priesthood had gotten more organized. A priest was chosen by his bishop. In addition to leading his congregation, the priest was the only one who could celebrate Communion and hand out the bread and wine to people. Priests were also the only people who could baptize people to make them Christians. (Priests didn't marry people yet at this time). Christian bishops tried to make sure that all priests could read and write Latin or Greek, but they couldn't always manage it, especially for priests in small villages.
In the Middle Ages in Europe, priests got control of more things than they had before. You needed a priest to be legally married, and to give you the Last Rites before you died so you could go to heaven. Priests were also responsible for collecting the tithe - a special tax that you paid to the Christian Church. on the other hand, priests had to live by more rules than before too. In addition to being encouraged to learn to read and write, priests in western Europe weren't supposed to get married or have children, though in Eastern Europe that was still okay.