The Sacrament of Holy Orders is the continuation of Christ's priesthood, which He bestowed upon His Apostles.
In the Sacrament of Holy Orders, a man is incorporated into the priesthood of Christ at one of three levels: the episcopate, the priesthood, or the diaconate. The priest being ordained vows to lead other Catholics by bringing them the Sacraments (especially the Eucharist), by proclaiming the Gospel, and by providing other means to holiness.
The Sacrament of Holy Orders can be validly conferred only on baptized men, following the example set by Christ and His Apostles, who chose only men as their successors and collaborators. A man cannot demand ordination; the Church has the authority to determine eligibility for the Sacrament.