What settings help in getting a good bokeh along with a nice foreground image?

Example -

I have 18-105mm lens and a D90 Nikon.

2nd QUESTION -

Why are sometimes bokeh round & sometimes Octagonal or Pentagonal etc?

Thanks again!

Added (1). Thanks! Great answers!

You want to use a large aperture and a small f/stop.
And i don't know I guess it depends on the settings.

To have a nice bokeh effect in a photograph, you need to have a shallow depth of field. A larger aperture will produce a nicer bokeh than a smaller one will, so a 50mm f/1.4 will work better than the 18-105 kit lens that comes with the D90. Extend your lens to its longest focal length; get as close to the subject as possible. The further away from the subject the background is, the better the bokeh will be.

The different shapes of bokeh are caused by the shape of the aperture. Lenses are all made differently; some have more blades in the aperture, some have less. Those variations affect how the bokeh appears. You can even use lens covers with shapes like hearts or balloons to change the shape of the bokeh in your photographs.

You want a large aperture - that means the hole must be big so the ƒ stop must be a small number like 1.8 or 2. I think that is what the person above was attempting to say.

Focus on the object in the foreground, open up as wide as the lens will go and set the shutter accordingly. You will put the background out of focus. This is limited depth of field, not true bokeh.

As you noticed, some bokehs are round, others octagonal, et cetera. This is true bokeh, and it is created by the interaction of light and the lens diaphragm. For a detailed discussion of both depth of field and bokeh (circles of confusion) see this site:

http://www.cambridgeincolour.com/...-field.htm