First of all if you are getting paid money then no you and your camera are not ready, and if you are not getting paid money, you need to advise the bride that they need to find a competent professional, please, please do not do this. The D5100 is a nice camera but it is hardly one to be used for weddings. When doing weddings you need two of everything and I mean two if not three of everything including cameras, lenses, flash units, batteries, chargers, even cables and cords because not only can Murphy show up at your wedding but will.
When doing weddings also you will need fast lenses and cameras that can handle them, the D5100 is too small to balance most fast lenses. You also have to understand lighting and how to use multiple flashes and also when not to use them and how to use available lighting when flash photography is not allowed in some churches. Professional cameras, fast lenses, flash units all adds up to $$$$ and we have not even started to talk about post production software and editing.
If you are trying to do someone a favor and they cannot afford a professional, please do not ruin their day, do them a favor and offer to help them pay for one, just because you have a nice black camera does not mean you can step in and do the job, wedding photography is very, very difficult to do it right and good pros do make look easy maybe but it is not, it is very hard to do, very very hard in fact. You have to know your equipment and have a good solid background with lots of experience. Usually after learning the ins and outs of proper lighting and acquiring good equipment you can then begin to look for work as a second shooter under a seasoned professional and then ultimately get the required experience to go out on your own.
And further just for fun, please view this,
[Video] Judge Brown Tears Cheap Wedding Photog a New One | Fstoppers. Hopefully it drives home the point.
Rob