As a reader you will be performing a service, but you do not need to be a professional performer to do this. Rather, it is more important to allow others to hear the words and layers of meaning. Don’t assume a pious or lofty voice; speak slowly and clearly with the rhythm and voice that you usually use. Pay attention to your listeners, do the guests near the back appear to hear you? Practice beforehand so the words are familiar to you. And most of all, relax. You are sharing in the joy and happiness and to be chosen to read at a wedding ceremony is an honor.