First up, take a look back through my posts tagged “wedding attire” and also Put This On’s guide here and here.
Secondly, what does the wedding invitation say? Usually, there will be a dress code. I’d take my cues from that.
Generally though, I’d wear something fairly conservative. Dark grey or navy suit (no pinstripes), white shirt, polished black shoes and a wedding tie. Maybe a lapel flower or small boutineer if you feel so inclined. A deep brown suit might also work if the wedding is more casual, however, I don’t think brown works terribly well in a formal setting during the evenings.