Fish Eye and Crop Factor

So I want to get a Fish Eye Lens to do some photography and I was talking to a guy about getting the right one for my camera. He talked about Crop Factor and Sensor Size and, well I have no idea what lens to get. I have a Nikon D3100 and the senor size is 23.1 X 15.4 mm. What's a good size fish eye lens to get for this camera?

Thinking back to when I had a fisheye (for a full-frame film camera) it was 8 mm, but 6 mm were also popular. Taking into account the cropped frame, I'd say that you would need 6 mm or shorter, if you can get one.

Fisheye lenses on cropped sensor versus full frame cameras.http://lensesforhire.blogspot.co.uk/2012/04/which-fisheye-lens.html - all the info you need is there + pictures + diagrams.