There are two routes depending upon if the Canadian in the U.S. or in Canada: 1) adjustment of status or 2) she leaves the U.S. and they do consular processing.
If she is in Canada then there are two routes: 1) U.S. Citizen marries her in Canada [if this is a same-sex marriage, make sure that Canada allows it], then petitions her and she consular processes or 2) Fiancee visa (K-1).
Answered on May 23rd, 2016 at 3:43 PM