Think about the three key moments: walking down the aisle, the signing of the register, and walking back out. Decide whether you want a classical feel, something contemporary, or a mix, and check the acoustics and any music restrictions at your church or venue. Listen to recordings before you book.
Ask which songs are in their repertoire, whether they can learn a special request, how much space and time they need to set up, and whether they can also play during the drinks reception.