Marriage is a "covenant or partnership of life between a man and woman, which is ordered to the well-being of the spouses and to the procreation and upbringing of children. When validly contracted between two baptized people, marriage is a sacrament".(Catechism of the Catholic Church, 1601)
If you are planning a wedding, your first step is to be in touch with a priest, so you and your fiancé can meet with him to begin the preparation process which includes several meetings with him, paperwork to be completed, and a preparation class. Marriage prep classes are hosted in the Archdiocese of Washington.