Sandrine, a belated welcome to the forums! I didn't realize this was the beginning of your stay with us.
You've gotten some excellent advice already, and the link that Miserere provided is incredible. I wish I had read something like that before I did my first wedding! I think all the things you need to know are there; the only things left to decide are the parts that are going to determine your style so you can leave your own mark.
You see, I don't feel comfortable with zooms, so I usually shot with prime lenses, including (gulp!) at weddings. That's why it makes sense for me to have multiple bodies. My most successful wedding to date - the one that had the shots I was happiest with - were taken with two K100Ds and one K200D (I'm all about the AA batteries) and the FA 31, FA 43, DA 70, and the D-FA 100 macro. I knew that the bride didn't want any group shots (make sure to get a list of all the shots they want - in fact, communication in all respects is key to making sure it goes smoothly and everyone ends up happy), so I didn't bother bringing anything wider. I do 95% of all my shooting between 31mm and 100mm, so this worked out perfectly for my style.
The last wedding I shot was in this fantastic tiny church with no natural light at all. I'd scouted it out beforehand, so I knew to bring my widest rectilinear lens - the 12-24 - but even then it was almost impossible to get the huge groups that the bride wanted.
Good luck, and let us know if you have more questions!