Well, smile, be pleasant, and you should have no troubles honestly.
Guys really aren't like girls, we don't expect gifts or anything.
As far as doing something? What could you do? The only thing I can think of us to bring some type of food . . like muffins or something. I am not sure why that psrings to mind lol.
And thats assuming you are going to the Fraternity House. Honestly, most of the Brothers will be cool just because you are one of their brothers' GF's.
So be of good cheer and don't be nervous