How to auto foucus on a nikon 3100?

I now how to record but i've been trying to record my fish tank but it so blurry cause i don't know how to Focus the camera can someone help me please. I want to know how to auto Focus it or manually focus it.id really love it you give me clear details on how to do it thanks you!

Let me see, I'm sure there was a small book that came with the camera. If you can read, do so

The owner's manual that came with your camera covers these things. Stop being lazy and read it.

"How does RTFM get so many thumbs-up? The minimum focus distance isn't even a camera issue - it's lens related".

Not very helpful.