How do I take a picture with a blurred background with my canon eos 70D using the 18-55 mm lens
This is a usually situation for portrait. The important is make the most short possible the dept of field in your lens and this you will do controlling the aperture. Because your question, I guess you are new in photography and I think you must read some theory for you can enjoy better your excellent camera.
- set the camera in Av. this will let you control the aperture and the camera will adjust the speed.
- set the minimum F number your lens offer you. Big F number close the diafragm, less light come to your sensor but more depth of field you will have and subject and background will be in focus. It is the oposite you want, so, go to the small number :)
- zoom and make the composition of your photo
- important: when zooming the F number will change, maybe at 18mm you will have 2.8 but at 55mm you will have 4.5 because the focal length affext the aperture.
- you didn't ask, but the focus mode is important: if you will shoot to some subject moving with short depth of field will be more complicate get the right focus
- if is a portrait, the focus go to the eyes of the subject
- if you can get other lens, I recommend around 70mm for portrait. Go little far from your subject and zoom in, with small F numbers (big aperture) you will have the best results
- be carefully with flash! You will have big aperture, and white people with a strong light can 'burn' the picture
- REMEMBER: big aperture == small F number, small aperture == big F number. You will use the first case.