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Under the Law of Moses, the presiding officer of the Aaronic Priesthood was called the high priest. The office was hereditary and came through the firstborn among the family of Aaron, Aaron himself being the first high priest of the Aaronic order.
Ordination to the office of high priest generally occurs when a man is called to serve in a position that requires him to be a high priest. For instance, he may be called as a bishop, stake president, counselor to these men, as a member of the Stake High Council, or a member of a high priest's group leadership. It is the stake president who determines when a man should be ordained a high priest; he serves as president of the high priests quorum (each stake has one such quorum), with his counselors serving as counselors in the high priests quorum presidency.
In addition, each ward within the stake has a high priests group from whom a leader is called by the stake president. That group leader then selects two assistants, along with a secretary, who assists him in managing the affairs of the group. These affairs include home teaching and other responsibilities delegated by the Bishop, who is the Presiding High Priest within the ward.