It depends on your relationship to the couple who is getting married. Are you related to one of the two? Are you friends with the couple?
If you aren't close with the couple, a simple congratulations should be sufficient. If you are close with either of them, something more heartfelt would be in order. For example if you are related to the bride, a little note welcoming the groom* to the family would be appropriate.
If you are friends with one (or both), a few kinds words about how glad you are they found each other would be nice.
All in all, it depends on your relationship w/ the couple. No matter what, keep it nice.
*or the other bride, if that's the case