Lisa - I've gone through this myself. Shooting indoors without a flash isn't easy, and gear does make a difference. And the more available light, the better. So, if the church has lots of windows, if it's a sunny day, if the wedding isn't at night, etc - all these things will make your life easier.
I'd stay with aperture mode. That way, if you set your ISO at 800 (or higher), then the shutter speed will be as fast as possible under those circumstances.
Keep the lens as wide open as possible, an remember that zooming in will close down your aperture and cause you to lose light.
Does the D3000 have a CMOS sensor? If so, you should be able to get good results with ISO of 1600 and maybe even 3200. The extra stop or two could be quite valuable.
Also, be careful about a narrow depth of field. So, for example, if you're photographing a row of people form an angle, then you'll likely have a hard time getting the whole row in focus. Better to shoot from straight on if possible.
Any chance you can borrow or rent a fast prime or a fast(er) zoom?
Finally, at least in my church, we have many different light sources: sun, sun through stained glass, tungsten, florescent, and areas where all these blend together. If your church is similar, for this reason alone (and in case you underexpose some photos), shooting in RAW might be a very good safety net for your photos of the ceremony.
Good luck.
