If you are a US citizen and you get engaged to a non-US citizen, you need to secure the right visa for your fiancé to be allowed to enter the US for your marriage. This visa is the K1 Visa which is a nonimmigrant visa. Only a certified US citizen is allowed to file for the K1 visa on behalf of his or her fiancé.
Since you are the US citizen, you will have to also submit a duly accomplished Form I-129F to the USCIS Dallas Lockbox in connection with your fiancé’s visa application. Send your documents to the following address: USCIS P.O. Box 660151, Dallas, Texas 75266 through the US Postal Service. If you send it via courier or Express Mail, you must send it to the following address: USCIS, Attn: I-129F, 2501 South State Highway 121 Business, Suite 400, Lewisville, Texas 75067.
Once the K1 Visa has been approved, it is up to the National Visa Center (NVC) to handle your case. Your fiancé will then have to go through consular interviews. You will be issued an NVC case number unique to your case.
In some cases, it may be necessary to apply for K1 Visa Renewal. You may have to apply for K1 Visa Renewal also to upgrade your petition to include your fiancé’s underage children. You might also have changed lawyers in the interim so that would be another reason to change the case info. One reason for applying for K1 Visa Renewal is if certain information in your NVC case file has changed over time.