To get a blurred background, you want a large aperture. To set the aperture, you need to be in aperture-priority mode. If you have the kit lens, the best you can do is probably f/3.5 -- which is good, but not great, for blurring backgrounds.
Try setting the aperture to each of these values, take the same photo, and compare the results: f/3.5, f/8, and f/16. This will help you see what happens when you change the aperture.
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