Blurry Background Effect Nikon D3100?

I have a nikon D3100 & I make beauty videos on youtube, I want a lense that will focus on me, but that will give a blurry background effect.

Use the longest focal length you have, with the lenses aperture wide open. Finally place your subject as close to the camera as you can and as far away from the background as possible.

This will assure an out of focus background

You don't want to change the lense or add a filter, instead you need to get the depth of field right.

Depth of field refers to the depth (distance) within an image or video which is in focus within the field (everything that's in shot) So if everything in the shot is in focus, you have a large depth of field (DOF)(sometimes called deep focus), and if only the subject close to the camera for example is in focus, then you have a small DOF (shallow focus).

The distance from the camera to the subject you want to be in focus can be important. If you have the camera 20cm from the subject (close) and 8m from the background wall (far away), the subject and background are far away from each other and the subject is much closer to the camera than the background. In this instance it is easier to adjust the depth of field to focus on the subject and blur out the background.
If the subject is 6m from the camera and the wall an additional 2m away, the subject and the background are both distant, and are quite close to each other and both are far from the camera. This makes it much harder to control the depth of field to include only the subject and not the background. Both the subject and the background are at a similar depth compared to the position of the camera and so will both be in focus.

The depth of field is controlled by the aperture of the camera (in traditional photography and videography, this is the size of the hole which light is let through inside the camera) The smaller the hole (the higher the 'f' number on the camera settings), the deeper the depth of field. To make a smaller depth of field, you will need a lower f number setting if your camera has that option which will open up the aperture hole reducing the amount of light needed and also making the DOF shallower. Changing the aperture settings will change the amount of light you need (a smaller hole needs brighter light to stop everything looking dark etc…)

Some digital cameras offer 'zone focusing' which allows you to select which section of the field of view will be in focus.

The best think would be to do a little research on how to change the settings on your particular camera and see whether it has any settings you can use, and also experiment with camera and subject positioning in comparison to the background.