How to DSLR camera lens work?

I told my friend that I want to purchase a dslr.
I'm a newbie. And I was blown away by his answers.

I told him that basically I wanted the feature to be able to focus on 1 item with blur background.

I asked him about nikon d5000.

he told me that
1st the lens isn't that good - 18-55mm, he has Nikon D90 which have 18-105mm?

2nd he said something about it's lense - f/3.5

what difference does it make?

His lens is no better for the "blurred background" stuff than yours is. The part of the lens that determines this is the aperture, the changeable opening that controls how much light enters through the lens. This also controls depth of field, which in turn is what controls that blurred background.

Both the Nikkor 18-55 and the Nikkor 18-105 have variable apertures, which means the aperture changes as you zoom from 18mm to a higher number. Both of these lenses have an aperture range of f/3.5-5.6, and will perform pretty much the same in the blurred background category.

My turn to blow you away!

He is partially correct.

The degree to which the background is blurred depends on three things:
- wide aperture
- long focal length
- short subject distance

Since his lens is capable of a longer focal length, he can blur the background better than you can. That is partially compensated for by the need for a longer subject distance if one uses a longer focal length though.
Also, while both lenses are variable aperture ones, his will have a slightly wider maximum aperture at a given focal length. At 55mm his lens will open up to about 4 or 4.8 or so, while yours will be 5.6.

That being said, his lens isn't exactly good for shallow depth of field (as that which you want to achieve is called) either. For such purposes one would typically either use prime lens with a particularly wide aperture, e.g. A f/1.8 one, or a fixed aperture (usually f/2.8) telephoto lens.

Be aware that the typical budget portrait prime lens, the 50mm/1.8D one, won't autofocus on the D5000, as it's just a AF lens and not AF-S. One would have to get the 50mm/1.4G one for this then (G, not D!), which is nearly thrice as expensive but also even better.
Or you could get a D90 yourself… That one will autofocus with the 50mm/1.8.