How to get a good bokeh effect on Nikon d3100?

I'm hoping to figure out on how to do it on my d3100 with a 18-55mm or a 70-300 mm lens, I'm wanting to know how to do it for like out door portraits mostly. If not then what lens can I add on to either one for a bokeh effect?

For 'selective focus' shots where the background is blurred, but the subject is sharp, you need a narrow depth of field, for a narrow depth of field only two things you can control matter.

1) How close you are to the subject, the closer you are to the subject the narrower the depth of field, and…

2) Aperture, the wider the aperture (the lower the f number) the narrower the depth of field.

There are other things that reduce the depth of field sensor size being chief amongst them (the larger the sensor the narrower the depth of field), but as you can't alter that you're left with the other two.

Either lens may give you good selective focus at the longer focal lengths and wider apertures. In some bright settings you may find good results by using a and ( neutral density ) filter to allow wider aperture f-stop settings.

Also having the background far behind the person will help.

Practice, practice… To get better.

Maybe also buy a quality heavy pro style tropod is a great tool.

What's commonly called "bokeh effect" these days is simple a result of shallow depth of field from a lens that will allow it. A kit lens can't give you much in the way of bokeh. An effective and inexpensive lens for shallow depth of field is a 50mm f1.8.

You will get the shallowest depth of field shooting with the kit lens wide open at 55mm. Have the subject some distance away from the background.

Or, if using the 70-300mm, use it at the 300mm end, wide open.

I think you mean shallow depth of field (not bokeh).

read up on depth of field (things that affect it are aperture, lens focal length, and subject-to-camera distance)

Are you sure you mean bokeh? There's a large gallery of bokeh photos at bokehphotographs.com which you can look at for inspiration.