There is no visa petition for boyfriends and girlfriends, however if you are engaged to marry you can file a fiance petition. She can also try to qualify for any number of visas to visit the U.S., which do not depend on proving your relationship.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
If you plan to marry her, your best option is to file a fiancee petition on her behalf. If the petition is approved, she can come into the US to marry you within 90 days of her entry.