If you would like to eventually marry your girlfriend then you need to file form i129f and seek a fiancee visa for her.you will need to marry her within 90 days of arrival or she will have to leave.
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So long as you've physically met her within the past two years, you can file a fiance visa for her assuming you and she are willing and intend to tie the knot within 90 days of her entry to the U.S. That process will take about 9-10 months to get her here and upon getting married she'd need to file to get her green card. Or you can go to the Bahamas, get married and file an immigrant visa petition which would also take about 9-10 months to get here here but she'd enter the U.S. as a permanent resident (with a green card).
If you are planning to get married you petition for a fiancé(e) visa, if you can show that:
You (the petitioner) are a U.S. citizen.
You intend to marry within 90 days of your fiancé(e) entering the United States.
You and your fiancé(e) are both free to marry and any previous marriages must have been legally terminated by divorce, death, or annulment.
You met each other, in person, at least once within 2 years of filing your petition.
Good luck to you.
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