As the question does not specify what kind of a non-Catholic wedding (e.g. Protestant or Orthodox Christian, or Hindu?), the answer must of necessity be broad as well.
Be a respectful observer. Participate to the extent your own faith tradition allows. Pray well for the couple being married. Do what Jesus did at the wedding feast of Cana: add joy to the occasion.
Fr. Thomas Ryan, CSP is the Director of Ecumenical and Interfaith Relations for the Paulist Fathers.