There are a few basic requirements to sponsoring your fiancé's visa to come to the US to marry you.
1) You need to have met in person within the last two years. It doesn't matter where you meet - it can be here in the US, it can be in their home country, or you can meet somewhere else.
2) Your fiancé is over the age of 18.
3) You and your fiancé are both able to demonstrate that you are legally free to marry. Bigamy is not allowed in the US!
Those are the main requirements, and there may be some issues if either of you have a criminal history, so if you're interested in applying for a fiance visa for your beloved, you should consult with an immigration attorney.