R.A.
First, if it is in a church/chapel you need permission from the church to utilize their services( microphone, and if you need an accompanionment) Usually they will offer their services if you need assistance, so that should not be hard. IF you are using just background music , all you need is to supply them with the tape or cd, and they will make sure someone will be there to set it up for you . Schedule time to rehearse, and make sure everything is set up. Usually they charge a fee for a pianist or guitarist to accompany you.
The best thing is to contact the church, and they will give you everything you need to know about singing at the wedding.
I sang at my father's wedding,and his stepson's. I think it makes it a lot more intimate when you do it yourself then hire someone. Good luck!