Quote: I photographed my other 2 weddings with a fully automatic film SLR, so my next wedding will be my first with my new camera.
I take it that it is not your own wedding? We have discussed that recently.
Aperture is used to control depth of field, at one wedding I went to the professional photographer used f11. Everything was sharp! The results were dreadful. At the same event I was using between f4 and f5.6 and achieved the required result, sharp bride and groom and out of focus background. What you are trying to do is focus attention on the couple, not the venue.
The advice given above is excellent, learn about depth of field control elsewhere and then apply your knowledge to weddings.
I suspect that you will be "expected" to produce photographs and not doing so is not an option. With your camera set at ISO 400, f4 to f5.6 should give you suitable shutter speeds in most registry office conditions but, unless you can use flash, you may need to open up further to prevent camera shake form affecting your pictures. I doubt you will be taking many shots inside at the register office and those that you do take will be in circumstances where you can use flash. Do NOT be tempted to use a smaller aperture than f5.6 with flash indoors, every blemish on the walls will be crystal clear! If the camera tells you that the image will be over exposed, reduce the ISO.