How to focus my Nikon d3100?

How do I focus my Nikon d3100?
I'm trying to film a video of myself showing objects. But every time I hold an object up to the camera it doesn't focus on it, how do I focus objects to the camera not manually?

What's wrong with manual focus? It's the best type there's. You just have to know where the button is.

Are you sure the D3100 has autofocus? I have a Canon t3i so I don't know. Check online forums.

If it's on autofocus (where you switch from M to A) you press the button halfway through. But it does make a lot of noise.