How to Get a Green Card While Outside the United States
If you are currently outside the United States and are one of the specified categories of relatives of a U.S. citizen in a preference category, you can become a permanent resident through consular processing. Consular processing is when USCIS works with the U.S. Department of State to issue a visa on an approved petition when a visa is available. In this process the Department of State will issue you a visa. If approved, you may then travel on the visa and will officially become a permanent resident when admitted at a U.S. port of entry.
Getting married may or may not have tremendous consequences when it comes to immigration. If you are the unmarried son or daughter of a U.S. citizen and you get married prior to becoming a permanent resident, then you no longer qualify as an Unmarried Son or Daughter of a U.S. Citizen and will convert to the category of a Married Son or Daughter of a U.S. Citizen. This change in categories may result in a significant delay in your immigrant visa becoming available