How much does the nikon d3100 zoom?

Do you have to buy a special lense for it?

Uh… The zoom is determined by the lens.

The d3100 is a camera body. It does not zoom at all. It does not have a lens. You can purchase a stock kit which comes with a 18mm-55mm lens. This lens zooms 3X. You can later purchase any lens of diffrent focal lengths, zoom and prime to fit what you need to do with your camera. I suggest you a least view this video as well as take out books on photography from your local library. DSLR cameras do not take pictures. They allow you to take the pictures the way you want to. This requires a basic knowledge of lenses and exposure.

A DSLR is just a body - you buy lenses depending on the purpose of your image.

Forget about using such terms as 'zoom' when you are describing the reach of a lens for a DSLR - its a marketing tool used by point & shoot camera manufacturers, to impress customers & has no real relevance in the DSLR world.

Although you can buy 'zoom' lenses, lenses are more frequently referred to by their focal length. Quick run down of DSLR lens basics;

Prime lens - this is a fixed focal length lens - it doesn't zoom in & out, you move your feet to 'zoom'.
Zoom lens - this is a variable focal length lens - you twist a ring on the lens barrel to change the focal length.

Telephoto lens - has a long reach (brings distant subject closer). The focal length (in mm) will be big - anything above about 85mm. Good for sports/wildlife.

Wide angle lens - has a wide field of view so that you can get a lot in when you are close to the subject. The focal length will be small - generally below about 35mm. Good for dramatic landscapes.

Lenses in the 35mm to around 85mm are mid-range lenses. Good for general photography & portraiture.

Although you can calculate the magnification of a zoom lens (its the big number divided by the small number) is not real of any practical use. For example, an 18-200mm lens has a magnification of 11x.