In order for a marriage to be recognized by the Catholic Church as valid, the following must be met:
At least one of the parties must be a baptized Catholic. When two Catholics marry, the celebration of marriage normally is during a Nuptial Mass. A Nuptial Mass is a Mass which includes the celebration of the sacrament of marriage. It has special readings and prayers suitable to the Sacrament of Marriage. When a Catholic and baptized person of another faith marry, the rite for celebrating within Mass may be used. If either the bride or the groom has never been baptized, Mass is not permitted.
Both parties must be free to marry. This means that neither of them has a prior marriage that is recognized as valid by the Catholic Church.
They must be psychologically mature and capable of consenting to the marriage; and they must understand the nature of Catholic marriage as one that is exclusive, permanent, and open to having children.
If any of these requirements are missing, the marriage will not be recognized as valid and thus, the clergyman cannot witness the marriage for the Church.
Along with the above: The engaged couple must also meet several times with the priest/deacon who will be witnessing their marriage.
They must attend a marriage preparation program and receive a certificate of attendance.
Since preparing for marriage is a process, it is recommended that a couple begin a full year prior to the wedding.