Generally you want something with the widest aperture possible for low light shooting and to control depth of field. This would indicate something like the Nikon 17-55 f/2.8DX and the 70-200 f/2.8VR.
That said, there are many wonderful pictures taken with smaller max aperture lenses.It has more to do with your skill level once you have decent (instead of great) equipment. Drfrank has shown some great wedding shots taken with the 70-300VR Nikkor. I think you need to decide how much you want to spend, then buy the best you can within your budget.
When looking at lenses for this camera, remember that the lens must have a built-in focus motor (and AF-S lens in Nikonspeak) or it will not autofocus.
Welcome to Nikonians! Will you be shooting the wedding as a Guest or as the Pro? Do you have a budget in mind? What lens or lenses do you have now?
As Mick suggested above, the Nikkor 17-55mm f/2.8G ED IF AF-S DX combined with either version of the Nikkor 70-200mm f/2.8G ED IF AF-S VR (II) are the go to lenses for Wedding Pro's shooting DX bodies like your D5100.
There are third party options like the Tamron SP 17-50mm f/2.8 XR Di II VC LD Aspherical and the SP 70-200mm f/2.8 Di LD Macro that are significantly less expensive than the Nikkors.
Other options are the Nikkor 50mm f/1.8G AF-S, 85mm f1.8G AF-S, or even the 35mm f/1.8G AF-S DX which are all much less expensive, smaller, and lighter than the fast zooms. Good Luck and Enjoy your Nikons!
50mm 1.4g, 85mm 1.4g lens, if you can afford it, that 28-70mm 2.8 AF-S lens is awesome, if on a budget, try the Tamron 28-75mm 2.8, it does not have VR, but with a steady hand its a nice fast lens. The Sigma seems to be nice as well, but i have yet to give it a try. But i have taken some really nice indoor and outdoor shots with the Tamron and 50mm prime lens.
Except for the formals, I would never us a prime lens for shooting a wedding. While "zooming with your feet" sound like a good alternative to a good zoom lens, you will miss more shots and cause more disruption while you scoot around trying to get the shot you want.
My 2 lenses for shooting a wedding (with FX bodies) are the 24-120 f/4 VR and 70-200 f/2.8 VR. During the ceremony, I have each mounted on a body. There is barely enough time to switch bodies, much less mount a different lens or run up and down the isle chasing the wedding party.
The reception poses entirely different problems with space constraints and where the action is happening at that instant, compared to where you are or want to be.
Sorry to all you prime aficionados, but a modern wedding isn't the place for them. I have not seen one pro wedding photographer in recent years using anything but zooms, event for formals.