Whenever we attend Mass in our parish or elsewhere, we notice that the celebration of the Mass is always presided over by a priest or a bishop (when we go to the Cathedral or attend Chrism Mass). At other occasions we may see a deacon reading the Gospel or preaching or presiding at some liturgical gatherings apart from the Mass. Well, these men have all being ordained bishops, priests or deacons through the reception of the Sacrament of Holy Orders. This is a Sacrament of service, just like the Sacrament of Matrimony, and its minister is the diocesan bishop.
According to canon law (1032) anybody aspiring to the priesthood must have completed some required studies - in this case Philosophy and Theology. But first he must approach the Vocation's Director of the Diocese and be accepted into the seminary. It is only after the prescribed studies that he is ordained a deacon and then, sometime after, ordained a priest.
Some people choose to remain deacons permanently. These may be married if they choose.
The way people are called to the ministry differs. A very good starting point is to be prayerful and listen to your heart and, together with your Spiritual Director or the Diocesan Vocation's director - who are more experienced in such things, you can make proper discernment.