To achieve a blurry background or a bokeh effect you need a lens that has a wider aperture, like the prime 50mm f/1.8 or the 35mm f/1.8 lenses. They both have a wider aperture i.e. f/1.8 and the expensive ones even have more wider upto f/1.4 and f/1.2. The other prime 85mm f/1.8 and tele lens 70-200mm f/2.8 could also give to better bokeh effects with full blown up backgrounds, but with a big cost.
The wider the aperture the shallower the Depth of Field (DoF) will be. Means when the aperture is split wide open like at f/1.8 it can focus on a limited area and blurs the foreground & background. But when you narrow the aperture like to f/8 the focus area increases and the foreground & background also gets into focus. The more you narrow the aperture like to f/16 focus area increases, but at the cost of light and effects the quality of image. So, at wider aperture like f/1.8 more light gets through lens and you get brighter and clear images. But at narrower aperture like at f/8 or f/16 less light gets thru lens and the images gets dull. Check more about [Aperture here].
–> To simulate this with your 18-55mm kit lens you need to narrow down your DoF (Depth of Field) by placing the background farther to the subject in these simple steps:
1. Zoom to max 55mm focal length (if you have 55-200mm then zoom to the max focal length i.e. 200mm).
2. Set it to widest aperture, at 55mm you will get just f/5.6 stop.
3. Get as much as closer to your subject but make sure you frame it well and have the subject in focus.
4. Make sure the background of the subject is farther as it can.
5. The ideal ratio of fore and background should be 10:90
… Take few shots, play with the subject fore/back-ground gap, and choose the best shot.
–> Check these photos, I’ve taken them with a 18-55mm kit lens with my Nikon D5100, by using above technique: