I had the zechut of setting up two of my friends and they will be getting married in two weeks
As a token of their gratitude they have asked me to be an eid under the chuppah
Is there any special needs to do before being an eid, does one have to go to mikveh etc
Are we not all considered tamai these days
Does it makes a difference that the chatan is Sephardi and the Kallah is Ashkenaz
Being honored as a witness at a wedding is most probably the most significant honor given, because a witness has to be a kosher Jew. Meaning that he doesn’t have any known aveiros d’orayso. Therefore it is a significant honor.
In order to be a witness you have to make sure that you are not related in any way to any of the sides, and if you have aveiros that you know about, especially gezel, do teshuva for those aveiros. Additionally, make sure that you indeed see everything that you are supposed to be watching. Other than that the mesader kiddushin will guide you as to what you have to do.
There is no need to go the mikvah, as being an eid is not an inyan of kedusha.
The fact that the chosson is Sefard and the kallah is Ashkenaz, is an issue that the mesader kiddushin will deal with.