Tips for video taking?

I'm currently using a Nikon D3100… I have seen other people upload clear videos on youtube with similar camera models like canon 550D but they're videos are not as noisy/grainy as mine… Is there any tips to take nice HD videos? Here's the link to a test video i made

Thanks

Added (1). I understand that i need more light. But i have seen other videos under similar conditions but having a much cleaner clip…

You need more light. Try shooting outdoors under the sun.

The camera should be on a tripod or other solid mount, not in your hands.
With video, you shoot clips. In your sample, you would shoot the beginning clip, stop the camera, set up for the next shot, focus, and only then record. Stop, setup on the side of a face with glasses, focus, THEN record. And so on. Never waive the camera all over the place, and unless there's a good reason to have it on, turn off the autofocus when shooting.

And you need a LOT more light.