Before a man can be ordained as a deacon and priest he must be instituted as an acolyte (Code of Canon Law, canon 1035).
But becoming an instituted acolyte is not just a step to the priesthood. A bishop can institute a man as an acolyte who has not intention of becoming a priest. A man can intend to become a priest, be instituted and then change his mind about ordination. He remains an instituted acolyte and is free to marry.
An important document on instituted acolytes is the 1972 Motu Proprio “Ministeria Quaedam”. It is at romanrite.com/Churchdoc.html and includes:
“2. What up to now were called minor orders are henceforth to be called ministries.
3. Ministries may be assigned to lay Christians; hence they are no longer to be considered as reserved to candidates for the sacrament of order. …
6. The acolyte is appointed in order to aid the deacon and to minister to the priest. It is his duty therefore to attend to the service of the altar and to assist the deacon and the priest in liturgical celebrations, especially in the celebration of Mass; he is also to distribute communion as a special minister when the ministers spoken of in the Codex Iuris Canonici can. 845 are not available or are prevented by ill health, age, or another pastoral ministry from performing this function, or when the number of communicants is so great that the celebration of Mass would be unduly prolonged. In the same extraordinary circumstances an acolyte may be entrusted with publicly exposing the blessed sacrament for adoration by the faithful and afterward replacing it, but not with blessing the people. He may also, to the extent needed, take care of instructing other faithful who on a temporary basis are appointed to assist the priest or deacon in liturgical celebrations by carrying the missal, cross, candles, etc., or by performing other such duties. He will perform these functions more worthily if he participates in the holy eucharist with increasingly fervent devotion, receives nourishment from it, and deepens his knowledge about it.
As one set aside in a special way for the service of the altar, the acolyte should learn all matters concerning public divine worship and strive to grasp their inner spiritual meaning: in that way he will be able each day to offer himself entirely to God, be an example to all by his gravity and reverence in church, and have a sincere love for the Mystical Body of Christ, the people of God, especially for the weak and the sick.
7. In accordance with the ancient tradition of the Church, institution to the ministries of reader and acolyte is reserved to men.”
Another important document is the 2002 General Instruction of the Roman Missal (GIRM). From the USA editon, which can be accessed at romanrite.com/girm.html (without the footnotes):
“The Ministry of the Instituted Acolyte and Lector
98. The acolyte is instituted to serve at the altar and to assist the priest and deacon. In particular, it is his responsibility to prepare the altar and the sacred vessels and, if it is necessary, as an extraordinary minister, to distribute the Eucharist to the faithful. 84 In the ministry of the altar, the acolyte has his own functions (cf. below, nos. 187-193), which he must perform personally.”
“C. THE DUTIES OF THE ACOLYTE
187. The duties that the acolyte may carry out are of various kinds and several may coincide. Hence, it is desirable that these duties be suitably distributed among several acolytes. If, however, only one acolyte is present, he should perform the more important duties while the rest are to be distributed among several ministers.
The Introductory Rites
188. In the procession to the altar, the acolyte may carry the cross, walking between two ministers with lighted candles. Upon reaching the altar, the acolyte places the cross upright near the altar so that it may serve as the altar cross; otherwise, he puts it in a worthy place. Then he takes his place in the sanctuary.
189. Through the entire celebration, the acolyte is to approach the priest or the deacon, whenever necessary, in order to present the book to them and to assist them in any other way required. Thus it is appropriate, insofar as possible, that the acolyte occupy a place from which he can conveniently carry out his ministry either at the chair or at the altar. The Liturgy of the Eucharist
190. If no deacon is present, after the Prayer of the Faithful is concluded and while the priest remains at the chair, the acolyte places the corporal, the purificator, the chalice, the pall, and the Missal on the altar. Then, if necessary, the acolyte assists the priest in receiving the gifts of the people and, if appropriate, brings the bread and wine to the altar and hands them to the priest. If incense is used, the acolyte presents the thurible to the priest and assists him while he incenses the gifts, the cross, and the altar. Then the acolyte incenses the priest and the people.
191. A duly instituted acolyte, as an extraordinary minister, may, if necessary, assist the priest in giving Communion to the people. 100 If Communion is given under both kinds, when no deacon is present, the acolyte administers the chalice to the communicants or holds the chalice if Communion is given by intinction.
192. Likewise, when the distribution of Communion is completed, a duly instituted acolyte helps the priest or deacon to purify and arrange the sacred vessels. When no deacon is present, a duly instituted acolyte carries the sacred vessels to the credence table and there purifies, wipes, and arranges them in the usual way.
193. After the celebration of Mass, the acolyte and other ministers return in procession to the sacristy, together with the deacon and the priest in the same way and order in which they entered.”