How to focus on a Nikon d3100?

I just got my camera and went to take a photo and it will not autofocus. Please help.

Added (1). There isn't instructions o. O

If you just got your camera, then the first thing you need to do is read the instructions that came with the camera… There could be several reasons why it will not autofocus, and all of those reasons are in the instructions.

Download the instructions from the Nikon website…

What do you mean there isn't instructions. Unless I'm mistaken Nikon packed an owner's manual with the camera along with a software disk. Perhaps this would be a good time to check the box, huh?

Read the manual that came with your camera. If none came,
http://support.nikonusa.com/app/answers/detail/a_id/16910/~/user's-manual---d3100---guide-to-digital-photography
digital copy makes it even easier to find things.

Ill give you a hint. Either it is the lens, or the camera body!

Switch the A/M switch on the lens to A