How to film on a nikon d 3100?

I have a nikon d3100 and i want to know how to film videos on it.

RTFM.

Start reading on page 45 of your user manual By the time you get to page 51, you will have a good idea how to use the video feature on your camera

You can't "film" video. That would be like trying to swim across a road.

Video is video. Film uses film. To shoot film, you need a 16 mm or 35 mm motion picture camera. You can shoot video using a true video camera or a still camera with a video feature.

I have never seen a Nikon D3100 so this is a guess based on a brief encounter with a D5100.

1 Switch to live view
2 Press the little red record button by the shutter release.

fhotoace has clearly never visited the UK
Because this summer it has been perfectly possible to swim across many roads and, in common with many Amercians, he fails to realise that we use some words differently over here and takes more space correcting people's use of English than answering the question - rather like I have just done:-)