Depends on your budget, but you're going to want something in the telephoto range. I'm guessing because you've bought a D3100 (like myself), you're not looking to spend a lot of money. I'd suggest something along the lines of the
Nikon 55-200mm VR. It'll run you about $200 on Amazon. You could also go with the
55-300mm VR which will get you a little closer zoom, but will cost you about $360.
The unfortunate part is that neither of these lenses have great apertures (they max out at f/4 and f/4.5, respectively, and get worse the further you zoom in. So shooting in relatively low-light situations (i.e. orchestra concerts) becomes a bit more difficult.