If she can come to the US on other visa, you may marry her in the US and file immigrant petition with the USCIS. It may take about 6-8 months depending on the location to receive interview notice.
If she cannot come, you can either file fiancée visa or if you marry her you can file I-130 first, after approval she will get an immigrant visa interview in her country (US Embassy).
You should consult with an immigration lawyer for advice and possible representation.