Does anybody know whether there is a certain prayer the priest must say for me to wear this scapular? Or does he only have to bless it? Also, does anybody know the prayers? Are there any other requirements I must follow? Thank you!
Yes, as with the wearing of most scapulars, it would appear that there is an investment ritual involved:
The Scapular of St. Michael the Archangel
While this scapular originated under Pius IX, who gave it his blessing, it was first formally approved under Leo XIII. In 1878 a confraternity in honour of St. Michael the Archangel was founded in the Church of St. Eustachius at Rome, and in the following year in the Church of Sant’ Angelo in Pescheria (Sancti Angeli in foro Piscium). In 1880 Leo XIII raised it to the rank of an archconfraternity, which was expressly called the Archconfraternity of the Scapular of St. Michael. At first (1878) the confraternity received indulgences from Leo XIII for seven years; the summary of indulgences of the Pious Association of St. Michael was last approved for ever by a Decree of the Congregation of Indulgences, 28 March, 1903. The scapular is so associated with the confraternity that each member is invested with it. The formula for blessing and investing with the scapular, given in the Rituale Romanum was first approved by the Congregation of Rites on 23 August, 1883. In outward form this scapular is different from the others, inasmuch as the two segments of cloth have the form of a small shield; of these one is made of blue and the other of black cloth, and of the bands likewise one is blue and the other black. Both portions of the scapular bear the well-known representation of the Archangel St. Michael slaying the dragon and the inscription “Quis ut Deus”.
(from New Advent Catholic Encyclopedia: newadvent.org/cathen/13508b.htm )