1520s, "an accursed thing," from Latin anathema "an excommunicated person; the curse of excommunication," from Ecclesiastical Greek anathema "a thing accursed," a slight variation of classical Greek anathāma, which meant merely "a thing devoted," literally "a thing set up (to the gods)," such as a votive offering in a temple, from ana "up" (see ana-) + tithenai "to put, to place," from reduplicated form of PIE root *dhe-"to set, put."

By the time it reached Late Latin the meaning of the Greek word had progressed through "thing devoted to evil," to "thing accursed or damned." Later it was applied to persons and the Divine Curse. Meaning "act or formula of excommunicating and consigning to damnation by ecclesiastical authority" is from 1610s.

Anathema maranatha, taken as an intensified form, is held to be a misreading of I Corinthians xvi.22 where anathema is followed by Aramaic maran atha "Our Lord hath come" (see Maranatha), apparently a solemn formula of confirmation, like amen; but possibly it is a false transliteration of Hebrew mohoram atta "you are put under the ban," which would make more sense in the context. [Klein]

So it precedes Christianity, and I'll posit that 1 Cor 1:12; 3:22 is the first reference with the heretics being unnamed.

If you wish a more narrow definition of being officially labelled anathema, I would search for anathema either in Church documents or in a more focused first seven church councils. That brings you to:

CANON XXXIX

Of the care and power which a Patriarch has over the bishops and archbishops of his patriarchate; and of the primacy of the Bishop of Rome over all.
Let the patriarch consider what things are done by the archbishops and bishops in their provinces; and if he shall find anything done by them otherwise than it should be, let him change it, and order it, as seemeth him fit: for he is the father of all, and they are his sons. And although the archbishop be among the bishops as an elder brother, who hath the care of his brethren, and to whom they owe obedience because he is over them; yet the patriarch is to all those who are under his power, just as he who holds the seat of Rome, is the head and prince of all patriarchs; in-asmuch as he is first, as was Peter, to whom power is given over all Christian princes, and over all their peoples, as he who is the Vicar of Christ our Lord over all peoples and over the whole Christian Church, and whoever shall contradict this, is excommunicated by the Synod.1
[I add Canon XXXVII. of Echellensis’s Nova Versio LXXXIV. Arabic. Canonum Conc. Nicæni, that the reader may compare it with the foregoing.]
Let there be only four patriarchs in the whole world as there are four writers of the Gospel, and four rivers, etc. And let there be a prince and chief over them, the lord of the see of the Divine Peter at Rome, according as the Apostles commanded. And after him the lord of the great Alexandria, which is the see of Mark. And the third is the lord of Ephesus, which is the see of John the Divine who speaks divine things. And the fourth and last is my lord of Antioch, which is another see of Peter. And let all the bishops be divided under the hands of these four patriarchs; and the bishops of the little towns which are under the dominion of the great cities let them be under the authority of these metropolitans. But let every metropolitan of these great cities appoint the bishops of his province, but let none of the bishops appoint him, for he is greater than they. Therefore let every man know his own rank, and let him not usurp the rank of another. And whosoever shall contradict this law which we have established the Fathers of the Synod subject him to anathema.2

Philip Schaff and Henry Wace, eds., “The Captions of the Arabic Canons Attributed to the Council of Nice,” in The Seven Ecumenical Councils, trans. Henry R. Percival, vol. 14, A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series (New York: Charles Scribner’s Sons, 1900), 48.

You will then have to read histories of the council to determine who this is targeted against i.e. why it was an issue .... I've got to leave you some of the fun.