As marriage is a sacrament, these are the rules Catholics need to follow: A man can get married only with a woman of the same faith, because they both need to receive the communion during the celebration of the wedding. Exceptions can be granted under a specific canonic permission to do marry waiving this requirement issued by the Church. The couple also commits to raise their children under the Catholic faith.
For Catholics, the wedding is the only sacrament in which the priest is the Church's witness instead of the president. Groom and bride get together before him, godparents and their witnesses, and the Bible is read as any other common mass. After a short preach, the priest asks the couple about their will of getting married and to exchange vows if both answer affirmatively. With the exchange of the rings, the wedding is considered sealed.
After that, the ceremony continues as any standard mass, including the communion of the couple and the rest of the attendants.
That depends because current Catholics do not behave as strictly with their religion commands nowadays. If you meet a Catholic single who does not live his faith very strongly, I'd say chances are really high; if you meet a more consequent Catholic, I'd say it is almost impossible.