It is possible to get a green card by marriage if you are married or will marry a US citizen. However, the process for getting a green card by marriage can differ widely, depending on your situation. The biggest factor is whether a couple are already married and where the non-US fiancé or fiancée of the US citizen is living. If the fiancé/fiancée or spouse of a US citizen is already living in the US, the US citizen can generally apply for a US green card for the spouse by filing Form I-130 (Petition for Alien Relative), and Form I-485 (Application to Register Permanent Residence or to Adjust Status) with the USCIS after the marriage. As long as the fiancé/fiancée or spouse of the US citizen entered the US legally, the new spouse can usually get a green card. However, if the spouse of a US citizen entered the US illegally, he or she may be ineligible for adjustment of status, even if they are now married to a US citizen. In these cases, it is best to consult with a good immigration attorney before proceeding further.
In many cases, a couple will decide to get married and will apply for a K-1 visa while the foreign-born fiancé/fiancée is in their home country. The K visa interview can take place at a US consulate or embassy abroad. If granted, the K-1 visa grants the fiancé/fiancée the ability to enter the US for the purpose of marriage. Once in the country, the couple have 90 days to get married. After the marriage, the new spouse can apply for a US green card and can adjust status right away. The advantage of this process is that the fiancé/fiancée enjoys the right to work in the US and live in the US right away. As well, since the consular interviews are handled up front, there can be less of a wait for the green card.
Either way, the green card process can take months or even years. Due to concerns about immigration fraud, every application for a green card by marriage is subject to scrutiny. Anyone who applies for a US green card this way is subject to an interview to help the USCIS determine whether the relationship is legitimate. Despite this, every year many couples successfully apply for a US green card this way. If you have become engaged to a US citizen, it is a good idea to discuss with your partner which immigration process might be best for you.