Menu about auto focus of nikon d3100 with 18-55 lens?

When I put the lens to A model, the lens can't focus by it self. I put it to M model, it can focus by my hand. The d3100 is new for me so I have no idea how to set up it on the menu. Could you tell me how to set the auto focus? Thank you~ Once the same case happened on my pentax k10d, which I solve it on menu.

By default the lens should autofocus when you have it in A mode. You may have switched the camera to manual focus somewhere in a menu, but that's hardly something you could have done accidentally.

Without knowing further details i'd reset the camera to factory settings and see if the lens autofocuses when it's set to A on the switch on its side. If it does not, then it's defective.

The usual AF/MF switch is on the front of the body, at the right side of the lens mount when facing the front of the camera. There's also an AF/MF switch on some lenses. To use AF it would be logical to set both switches to AF. If it still does not work, consult the manuals of both camera and lens.

Maybe you should read the instructions that came with your camera.