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!