here is a video workshop by Gordon Laing, Editor of Camera Labs....
http://www.dslrtips.com/workshops/How_t ... ield.shtml
And to take a photo like the one you have linked to:
1- put the lens to manual focus
2- use the closest focus point you possibly can (opposite of infinity)
3- use the largest aperture you can. (in your case f/3.5 or 5.6)
4- snap the picture
And Three tips for getting shallow depth of field
1- large aperture (f/2.8 and larger works best, larger meaning f/2.0, 1.8, 1.4, 1.2 and so on)
2- shoot telephoto. 70mm will provide you with a much better bokeh than 18mm.
3- distance from subject. the closer you are to your subject the more shallow the DOF. The further your subject is from your background the more it is knocked out.
Notice in this image I took earlier today...
1- I shot this image at f/2.2
2- I shot this image at 50mm not telephoto but on a cropped canon body its 80mm which is a sweet spot for portraiture.
3- I am very close to my subject, my subject is very far away from the background.
the perfect combination for shallow DOF (depth of field)
...hope that helps