The 18-55mm kit lens has a max. aperture range of f/3.5-5.6, which is a little small to achieve good blurred backgrounds. As a lot of others have pointed out, a 50mm f/1.8 is a very good option, it has a large aperture that will give a very shallow depth of field and good blurred backgrounds.
If that is not an option, you can still achieve a shallow DOF to some extent with your 18-55mm kit lens, by shooting at the largest possible aperture. Also, you need to go close to your subject and have the background at a (reasonable) distance.
To do this, following these steps:
- Switch to the Aperture priority mode on the mode dial. On the Canon EOS 550D, this is labelled as Av on the mode dial.
- Compose the shot - go close to your subject, zoom to the desired level, make sure the background is visible.
- Rotate the command dial located below the shutter release button to switch to the largest aperture value. Remember that a lower number means a bigger aperture. e.g. at a given focal length, f/3.5 is a bigger aperture than f/5.6.
- Focus on the subject and take the shot.
- Review the shot to see if you've got the desired blurred background. If not, try increasing the distance between the subject and the background. Also, you can try increasing the focal length as higher focal lengths should give you shallower DOF.
- Keep experimenting!
Try this in a garden with a small object (like a toy) as the greenery and foliage would make for a very nice blurred background. Place the object where the background is at a distance and there are no distracting elements in between the subject and the background.
Go close to the subject and shoot as given above. Experiment with different focal lengths, aperture values and the background distance. When shooting at higher focal lengths, you may have to shoot lower down the ground at the eye level of the subject to make the background visible.
Hope this helps...