The two factors that determine depth of field are aperture and lens focal length.
The combination of these two lets you control the degree of background blurring.
You can control aperture by selecting aperture-priority auto exposure mode. The camera will automatically adjust exposure as you change the aperture. Choosing a low f-number softens backgrounds; in other words, using a wide-aperture lens (a faster lens) increases background blur.
The second factor is focal length: the longer the lens, the greater the background blur. So for a movie with backgrounds that are highly blurred, you need a long focal length at the widest possible aperture.
You should note, however, that the shallower the depth of field, the harder it is to maintain focus on the main subject. These settings are not the best choice when you need to track a moving subject. Adjust depth of field to suit the subject.
To adjust aperture, choose mode P, S, or A. Keep the exposure compensation button pressed and rotate the main command dial to choose a setting. Choose lower f-numbers to decrease depth of field, higher f-numbers to increase depth of field.