Samuel ministered, or served, not only as a priest but also as a prophet. As a priest he anointed the king, and as a prophet he assisted the king.
Samuel was established as a prophet by God (3:20), and no one could overthrow him.
Samuel was established by God to speak the word of God to replace the teaching of the word of God by the old priesthood. In the priesthood the first thing that a priest should do is speak for God. The breastplate and the Urim and the Thummim worn by the high priest were the means used by God to speak to His people. In the degradation of the priesthood, God's speaking was almost lost. Thus, God needed to raise up a living person, a prophet, to speak for Him, and this is what He did with Samuel. In God's ordination Samuel is counted as the first prophet. Of course, Abraham was a prophet speaking for God, but in God's ordained way Samuel was the first prophet to set up a prophethood.
In the Old Testament the last part of the divine revelation is with the prophets, from Isaiah to Malachi. Apart from God's speaking the universe would be empty. We thank the Lord that, by His mercy, we have His continual speaking in His recovery today.
Samuel's first prophesying was his speaking to Eli (3:1-18). Samuel received the word of God and spoke it to the old Eli in the waning priesthood. This speaking should have gone forth through Eli, but he was waning, and God could not speak through him. Instead, God spoke through someone much younger.