Whenever I film a video with my Nikon d3200 the background is really grainy?

I really want to fix it! I have it at 100 ISO and I have A LOT of lighting. Will a new lens fix it? If so which lens?

I have just had a look at some sample videos from a d3200 on youtube - and there's no graininess in the background.

You can set the cameras ISO to 100 for stills, but your camera is stuck using speicfic shutter speeds based upon the frame rate you are using. Once the light becomes too low for a good exposure with your lens wide open, all the camera can do is automatically increase the ISO, thus causing noise in the dark areas of the scene.

You need to add light when shooting indoors. Replace the practical (lamps found in the scene) lamps bulbs with 250 watt bulbs and use a couple of 500 watt continuous lights to provide overall ambient light.

My guess is that you have not considered the need to add light when shooting indoors.

Shoot

A faster lens may help graininess. It is another way of getting more light to the sensor. If you have an f/2.8 or faster lens, like f/1.8, you should see less graininess.