If a couple has already married civilly, and now they wish to "sacramentalize" their union, this is called "convalidation." Click here to know what to do if you want to convalidate your marriage.
Some marriage situations are complex and require a formal annulment to be realized beforehand. An annulment is a declaration by the Church that a previous bond that occurred in the Catholic Church was never truly a sacramental bond. This is different from a divorce. A couple may be legally divorced and legally remarried, but in the eyes of the Church, if the bond that occurred in the Church is not annulled, the persons are still bound to each other. In these cases, it is best to make an appointment with a deacon to have him assess your personal situation and offer steps to be taken so as to 'sacramentalize' your present marriage situation.
Please note: any previous bond (marriage), even if it was civil (not in the church), or only lasted a day, is considered a legitimate bond and must be addressed during the preparation process.