How To Blur The Background of a photo?

WE brought Nikon D5100 recently, i would like to learn photography. I would like To Blur The Background on the photo. I searched different web sites but, i didn't get the point. If we're zooming f/3.5 is changing(It is zooming the image) but not blurring the background.could any one help me out of this.

any suggestions for online photography learning websites.

You need:

1) Large aperture lens.f/2.8 or bigger. OR

2) A lens with a long (200mm+) focal length. Even a slow, kit telephoto will produce background blur due to the focal length

3) More distance between the subject and the background.

Check this web page, as it shows you how to blur the background without a fast lens and even to blur it with a compact camera:

http://www.althephoto.com/concepts/selectivefocus.php

However, you will not be able to get much effect if you are using your 18-55mm kit lens. You need either a fast lens or a telephoto lens.

Your kit lens is a "variable aperture" lens, which is listed as a 18-55mm lens f/3.5~5.6. That means that f/3.5 is only available at the 18mm end of the lens, and if you go to the 55mm end, the lowest aperture you can use is f/5.6.

If you had even a modest lens such as a 55-200mm f/4~5.6, then if you uses 200mm and f/5.6, you would get a decent background blur, even at f/5.6.

This is because focal length is the most important aspect of background blur, as the above web page shows. For that reason, you can even get some background blur effect with some compact cameras.

Standing further away from the subject will give you more bokeh
other than that, your best bet is getting a lens that allows you a lower aperture
the lower the aperture, the more out of focus the background will be!

These shots were taken from the same spot.

The camera was on a tripod.

The lens was manually focused on the metal fence. Each shot had different * aperture * setting.

If you would like to read the technical aspect of this, try here (it's a dry reading.) http://en.wikipedia.org/...h_of_field

In real life, it's used for something like this shot

or something like this, to make the main subject stand out.