There are certain eligibility requirements. You as the petitioner must be a US citizen and must plan to marry within 90 days of your fiancée entering the United States. If either person was married before they must provide the divorce papers, death certificate or annulment papers to prove the previous marriage was terminated. You must have met the person at least once within two years for filing your petition.
Once you marry your spouse may apply for a permanent residence and remain in the United States while it is being processed. For more information on this process check out the US Citizenship and Immigration Services website.