I'm getting a Nikon D3100?

I'm getting a Nikon D3100. And I was just wondering if there are any fish eye lens that are compatible with this camera.

Yes there are.

But be warned: There are cheap $20 screw on adapter lenses which offer horrible image quality. The real fisheye lenses will cost $350 at the cheapest (entirely manual ones) and $500+ or so for the standard quality ones.

The D3100 can use either the Nikkor 10.5 mm fisheye or Tokina 10-17 mm fisheye zoom

You will have to save your pennies for a while. They are NOT inexpensive