How to make the background blurry?

I have an af-s nikkor 18-55mm

Shoot at 55 mm lens setting with the lens aperture wide open.

Next position your subject as close to the lens as it will focus. This will throw the background out of focus.

The easiest way to do this is to use a fast, long lens, still positioning the subject close to the lens while keeping the subject in focus

Three factors affect Depth of Field (DOF):

1) The focal length of the lens.
2) The f-stop (aka aperture) used.
3) The subject distance.

This site will teach you about DOF and how to achieve a shallow or deep DOF:
http://www.dofmaster.com/dofjs.html You can use the DOF Calculator to compute DOF for any combination of the three factors imaginable.

Although conventional wisdom is that a wide angle lens will give the most DOF there are situations where you can achieve a fairly shallow DOF with the 18mm end of your 18-55mm zoom.

18mm @ f3.5 focused on a subject at 5'-0'' our DOF will be from 3.76' to 7.48'. Anything from 1.24' in front of our subject to anything 2.48' behind it will be in focus. If we keep our subject 3' or more away from the background then the background will be nicely out of focus.

To show what effect changing just one of the three factors - in this case the f-stop - will have, lets use the same situation as above except at f16.

18mm @ f16 and focused on a subject at 5'-0'' our DOF will be from 2.01' to infinity. Anything from 2.99' in front of our subject to as far as we can see behind it - infinity - will be in focus.

It's really simple, just select the widest aperture.

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

Good luck! The site has many more tutorials and tips!