Hi Rhonda,

Welcome to Nikonians!
I assume your Sigma 28-80mm f/3.5-5.6 Macro with a Nikon F mount.
While it will mount and meter on a D5100, since it has a screwdriver type AF mechanism it will Not AF when mounted on a D5100.
The D40, D40X, D60, D3000, D3100, D3200, D5000, D5100, and D5200 do not have AF motors built into the camera body so they require lenses that have AF motors built into them: Nikon AF-S, Sigma HSM, and Sigma with Built in Motor, etc...

The Sigma 28-80mm was never that great optically to begin with. Add the fact that 28mm is not very wide on a DX body like the D5100 it would make more sense to get the Nikkor 18-55mm f/3.5-5.6G AF-S VR DX or one of the other kit lenses. They are all optically excellent.
Good Luck and Enjoy your Nikons!

Best Regards,
Marty