How to take a panorama picture on a nikon d3100?

How do you take a panorama picture on a nikon d3100?

Same as you would any other DSLR.

You shoot a series of images (preferably with the camera upright, i.e. In portrait orientation, to minimise edge distortion), with about a 33% overlap between frames, with the camera on a tripod and stitch the resulting images together.

You will need a software for that like Photoshop or Hugin. Hugin is a freeware which you can download over the internet.