How to defocus a background and how do you make it look deeper?

I have a Nikon D3200 with 18-55mm lens and 55-200mm telephoto zoom lens.

To be truthful you need more aperture but it can be done with what you have. Here's a good article on the subject.

http://en.wikipedia.org/...h_of_Field

Your best results will be with your 200mm @ 200mm and f/5.6.

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