Photograohy depth of field?

Hi everyone i have a question. I have a Nikon D3100 and love it. I have already bought 3 lenses and can't seem to get the Dephth of field effect i want. Please help me and tell me what the best lense would be for this type of photo background blurs.

i added a link so you can see what type of depth of field am trying to get. The image on the far right is what i'm amining to shoot.

Added (1). Lens i have

Nikkor 35mm 1:18G
Nikkor 55-200mm 1:4-5.6G
Nikkor 18-55mm 1:3.5-5.6G

You didn't say anything about the lenses you have for your D3100, but I'm hoping one of those is the 50mm f/1.8 or something with a similarly large maximum aperture.

You need to shoot with a wide aperture. Your subject should be closer to the camera than to the stuff in the background. It's easier to do with longer focal lengths, but you can get shallow depth of field with pretty much any lens. You need to be shooting in manual mode or aperture priority at the bare minimum.

Here is how that works

Your camera is just fine. All you have to do is learn how to use it

http://camerasim.com/camera-simulator/

You will have no problem getting the effect of the left photo with the lenses you have. That photo looks like it was taken with a 50mm focal lenth lens and around f5.6. But in the right photo the background appares closer and her face appears slightly flatter. So I would guess it was taken with about 135mm focal lenth and either f2 or f2.8 apeture. The only lens you have with that wide of an apeture is the 35 f1.8 lens. But as you focal lenth gets shorter you will need a wider apeture to get the bokeh effect. So if you focus on a close subject with the 35mm f1.8 lens and have plenty of distance between your subject and the background, you should get a simular bokeh effect. The other two lenses will not blur the backgrond much because the max apeture is too small. However you can cheat and use the 55-200mm lens if you use a extension tube. However by using an extension tube you will only be able to focus on closer objects. And probally only use the 200m end to photograph the upper 1/2 of a person.