If you are a US citizen and wish to marry someone who is not a US citizen or green card holder, you will need to secure a fiancee visa, also known as a k-1 visa, to allow your fiancé(e) to enter the US in order to marry you. The K-1 visa allows a non-US citizen to enter the US in order for a marriage to take place. Once the marriage has taken place, the K-1 visa holder can get a conditional green card, and can eventually apply for a green card and even for citizenship.
To apply for a fiancé(e) visa, you will need to file a petition on behalf of your future spouse with the USCIS. You will also need to file Form G-325A (Biographic Information) along with USCIS Form I-129F (Petition for Alien Fiance) with the USCIS. You will need to include any needed documentation and the filing fees for these forms. Once the USCIS has processed these forms, you will receive Form I-797 (first Notice of Action (NOA)), which shows that the USCIS has received your application. Once this happens, you will then wait for the second NOA. If you get this, it means that the USCIS has approved your petition. If your petition is not approved, you may need to try again or consult with a qualified US immigration attorney to get legal advice.
Once the USCIS approves the petition, they forward the information to the National Visa Center so that background checks can be run on your fiancé(e) and so that the file for the visa can be processed. Once all this takes place, then the petition is sent to the embassy where your fiancé(e) lives. Your fiancé(e) will then get a letter from the embassy, requesting certain documents. Your fiancé(e) will need to send some documents and bring some documents to the consular visa interview or the medical interview. Once your fiancé(e) has gone through both interviews successfully, he or she will be granted a K-1 visa. Once your partner has their fiancé(e) visa they must enter the US within 6 months of that visa being issued. Once in the US, you and your future spouse must get married within 90 days. Your new spouse should also apply for a Social Security number and should apply for a conditional green card as soon as possible so that he or she can start living life in the US.