How to take a video with a Nikon D3100?

I just got my Nikon D3100 and I'm not sure how you make videos with it, or if you can, so if someone could help me out, it would be really nice, thank you.

On Top of the camera you will see a role wheel. There you will have a video icon. Just rotate the wheel there and it will be in a video mode. There you will have many other options like "M", "P", camera, hand icon. Just rotate and you can learn to use it best.

Congrats on you D3100…

You just got the camera and have not read the instructions that came with it? You're kidding right?

On the top right of the back of the camera, there's a switch that you push right to engage Live-View. Once you've done that, and your eye isn't glued to the viewfinder, you'll notice that in the centre of aforementioned switch is a red button. Push it and video recording starts instantly.
You'll have to keep the trigger half-pressed if you want to autofocus, btw - assuming you're using it on Full Auto and without manual focus.