I thought I might contribute a reply since most of the answers here are either recommendations for a high budget, or somewhat technical for a starter.

Since you asked specifically about lens, typically more diffused background is achieved by either increasing focal length, or aperture (i.e. smaller f-stop).

There are limitations for either, but particularly focal length as it may radically affect your subject distance with respect to your camera.

If you are looking for a portrait lens, which usually demands most bokeh, I would recommend an 85mm F1.8.
50mm F1.4 is good too, but its a little short for my preference. Either or both lens should be under $500 used I believe. I don't use Nikon so I don't quite know.
If you're looking for wider, I believe the new Nikon 35mm F1.8 for crop body seems to perform well. I wouldn't mind using that as a single lens.

Other factors influencing bokeh may include:
1) presence of image stabilizing lens elements
2) Lens construction (some long zooms don't have good bokeh despite shallow depth-of-field, typically your constant aperture zooms)
3) Your sharpness and contrast settings (sharper and contrasty bokeh may look like... chunky peanut butter, as someone has mentioned).
4) Your subject distance from the background
5) The background itself (e.g. trees give you generally harsh bokeh)

If you prefer to really know in high detail, read up what influences depth-of-field, as many have mentioned.