filing the K-1 visa
the US citizen petitioner must file from I-129F with the Service and must provide evidence that the couple has a bonafide intent to marry within the 90 days that are provided by the visa; that there are no legal impedepents to getting married (like a previous marraige...with pending divorce); and that the couple has met in person at least two years previous to the filing of the petition; and you have to prove any criminal record of the petitioner.
upon entering the US
upon entering the US the couple most marry within the allowed 90 days...if not the K-1 visa will terminate and beneciary will be considered out of status. Once married the beneficiary could petition to adjust status to that of greencard holder. the beneficiary cannot adjust their status any other way than through a marraige petition by the original K-1 visa petitioner. When filing for green card the beneficiary will be granted conditional residency status for two years...which will then have to be removed in order to obtain permanent residency.