If you are a U.S. citizen but the love of your life is not, then getting married will be more challenging. If you wish to live together here in the U.S., you will first need to get them a visa.
A K-1 non-immigrant visa would allow them to travel here while still unmarried in order to get married.
Make sure you can comply with the visa requirements
To apply for this visa for your fiancée you will need to show proof that you can support them financially. Things like bank statements, tax documents and employment contracts will be crucial here.
You also need to prove your romantic relationship is genuine, not just an attempt to help a person get into the country. Dig out those old love letters between you and photos or videos of the time you have spent together. If you lived together previously in another country, joint rental or bank contracts could vastly improve your claims to legitimacy.
You must also be able to complete the marriage within 90 days of their arrival. So start planning things early.
What if I am not a U.S. citizen
If you are not a U.S. citizen, but hold a green card, then you have two options. Either work toward gaining citizenship, then apply for a K-1 non-immigrant visa. Or, if there is a way to marry your fiancé(e) in another country, do that and then apply for a CR-1 Spouse visa.
Whichever route you take, it is wise to take legal guidance to give yourself the best chance of success.