How to make my camera on manual focus nikon D3100?

I don't know how to change my cam from auto focus to manual?

Step 1: Read the instructions that came with your camera.
Step 2: Do what the instructions say.

There should be a button on the lens A-M. A being for Auto, M being for Manual. Lens, not camera.

There's a switch with an a and an m on it, switch it to m for manual or a for auto.

The camera does not have an auto-focus motor. As people have stated, switch it to M on the left side of the Lens, and I recommend switching vibration reduction to On for better performance.

In the future when you buy your next Lens, if it doesn't have auto-focus built in, the camera won't be able to auto-focus at all.

I also recommend only shooting in manual focus for best results with the d3100's video.