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My usual advice to people who ask about doing weddings is "Don't".
However, if you must, here are some thoughts. I don't know who is commonly using a 70-200mm, but it would not be my choice. A long focal length is fine for candid shots taken without the subject's knowledge, but unless your church is in a huge field where you can afford to back off, a long zoom is not much good for the group shots.
I could suggest some suitable lenses, some cheapish others not, but I am not going to recommend you rush out and buy them for this first occasion.
Stick with the lens you know, the 18-55mm. It is wide enough for most wedding groups and long enough for some unobtrusive close-ups. If you want better bokeh (blurry backgrounds), just move your groups and subjects away from backgrounds especially if it consists of foliage.
I could prattle on with other tips forever as I devoted 5 years of my life as a semi-pro wedding photographer. For a beginner I would suggest Program Mode and Auto white balance and ISO. However, as I don't know your skill set I am not going to advise you further. Instead I'm going to wish you good luck and remember to forget about your equipment and just think about what you see in the viewfinder.