We know that when Alma baptised Helam he already had the authority to baptise,do we have any record of who baptised Alma?
Alma was given the authority to ordain and baptize, “And it came to pass that Alma, having authority from God…”(Mosiah 18: 18) , but the Book of Mormon does not tell us where or from whom he received the priesthood or when he was baptized.
King Noah’s father Zeniff left Zarahemla, being desirious of returning to their first inheritance, the land of Lehi-Nephi. It is likely that some in Zeniff’s party would have held the prieshood. When the Lamanities came to battle, they remembered the Lord and called mightily upon Him to preserve them in battle, “for we were awakened to a rememberance of the deliverance of our fathers.” (Mosiah 9:17) God did hear their cries and did answer their prayers. Zeniff’s group left Zarahemla around 200 B.C. and we are told that Alma held the priesthood some 50 years later, which was 10 years after the death of King Zeniff. It is possible that the priests of King Noah held the priesthood as did those of his father Zeniff, prior to their apostatizing. In their book ‘Doctrinal Commentary on the Book of Mormon’, McConkie and Millett state: “It may be that his (Alma’s) office of high priest in King Noah’s court was an actual priesthood office, an office to which he had been formally ordained by one having proper authority, this in spite of the abominations of the day.”
Another possibility is that he was ordained by Abinadi. Also, he could have been ordained by a heavenly messenger as was Joseph Smith and Oliver Cowdrey. We are not told who he received the priesthood from, only that he had the authority from God to baptize and ordain other priests. He would have been baptized as well prior to receiving the Priesthood.