If your spouse or future spouse is in the U.S., there is no need to file a fiancée petition. However she or he has to have entered the U.S. Legally with no crimes or past visa problems. Like any immigration petition or application it is so very important to fill the forms out correctly and completely or you can face delay in the processing that can take years to complete.
A spouse in certain circumstances can still get a green card even though they did not enter the U.S. legally but the rules are tricky and there are certain requirements that have to be met which many people are not able to meet.
Fiancée Petitions are for your future spouse who is not in the U.S. You can usually bring your spouse to the U.S. from her country to get married. The law requires that you have met in person within the last 2 years before filing but there are special situations where this can be waived. If approved you must get married with 3 months from the time your spouse enters the U.S.
If your spouse has children from an earlier marriage or relationship there are certain advantages to filing a fiancée petition rather than marrying first if the child is over 18 years old or older.
In short, marriage is a big step and is even more difficult when having to file immigration papers. It is important to plan carefully on what has to be done for the next few years.
Please contact a licensed Immigration attorney before you act on your specific situation. Immigration Laws and Federal Regulations are constantly changing and the general information provided can change over time.