How to create motion blur on Nikon D5100?

How do i take a picture with motion blur such as traffic blur?

Use a long shutter speed… Duh.

Use a tripod and remote shutter release, then reduce the shutter speed. Start out at 1/15th of a second and go down from there.

The photo below was done at 1/6th of a second:

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You'll definitely need a tripod and for exposures longer than 30 seconds a remote release like the Nikon ML-L3. For exposures of 30 seconds or less you can use the camera's self-timer.

You also need to understand the Exposure Triangle - Aperture, ISO, Shutter Speed - because you can't just set a shutter speed without balancing it with the aperture based on the ISO you're using.
This site explains the Exposure Triangle:

http://www.digital-photography-school.com/learning-exposure-in-digital-photography

Although the subjects are stationary here are two examples taken using ISO 200:

100mm @ f11, exposure of 30 seconds.
200mm @ f11, exposure of 30 seconds.

I use and recommend this site for low-light exposures: http://www.calculator.org/...osure.aspx

I chose the Scene 'Distant view of city skyline or floodlit buildings' and based my exposure on ISO 200.

This will help you learn more about your Nikon D5100:
http://www.nikondigitutor.com/eng/d5100/index.html

You will need a relatively long/slow shutter speed, the exact value will depend on the amount of light, the speed of traffic, the focal length and the scene in question.

Also it will depend on what you want to achieve, for example, light trails generally call for longer shutter speeds (more than 5 seconds) than panning shots (typically 1/100).