How to blur background for Nikon D3100?

I'm new to photography, so I was wondering. On the Nikon d3100/d5000 how do you blurthe background? When I tried it out, I couldn't blur the background when I was manually focusing, OR autofocused.

What you are asking about is called Depth of Field (DOF). This site will teach you what it is and how to control it.http://www.dofmaster.com/dofjs.html

There are only three factors that affect your DOF:

1) The aperture used.

2) The focal length of the lens.

3) The subject distance.

A general rule of thumb is a wide angle lens (like the 18mm end of your 18-55mm zoom) will give more DOF and a longer focal length lens (like the 55mm end of your 18-55mm zoom) will give less DOF.

At the referenced site you can use the DOF Calculator to compute the DOF for any combination of the three factors imaginable.

Here is an example of a minimum DOF picture made with a 70-210mm zoom at 210mm using f11 at about 8 feet:

Here is an example of a maximum DOF picture made with a 21mm lens at f16 and pre-focued at 6 feet:

Here is something to think about - both of my examples were made with the camera mounted on a tripod. In fact, I believe a tripod is mandatory for landscapes like the 2nd.example.

These sites will also help you learn more about photography:

http://www.photonhead.com
http://www.illustratedphotography.com/photography-tips/basic
http://www.digital-photography-school.com
http://www.cambridgeincolor.com

People usually associate aperture with the blurring of the background. The larger the aperture (smaller the number means a larger aperture), the blurrier the background. There are other factors associated with bokeh (blurry background) as the user above has pointed out.

Open your aperture and voila!

Here's a tutorial called "How to Blur the Background - http://www.the-dslr-photographer.com/2009/11/how-to-blur-background.html

Good luck! The site has many more tutorials, tips, reviews and guides!