How to Select Live View Focusing Pairs with a Nikon D3100
The way the Nikon D3100 sets focus when in Live View mode and video shooting depends on your Focus mode and AF-area mode settings. If you use the kit lens (or a similar lens), you also need to set the switch on the lens barrel to either A for autofocusing or M for manual focusing.
Until you get fully acquainted with all the various combinations of Focus mode and AF-area mode and can make your own decisions about which pairings you like best, try the following settings:
For moving subjects: Set the Focus mode to AF-F and the AF-area mode to Wide Area. You also can try the Subject Tracking AF-area mode, but remember that the camera usually can't track subjects that are moving very rapidly, are small with respect to the rest of the scene, and so on.
For stationary subjects: Set the Focus mode to AF-S and the AF-area mode to Wide Area. Or, if you're shooting a portrait, give the Face Priority AF-area option a try. Note that in a group shot, the camera usually locks on the closest face.
For difficult-to-focus subjects: If the camera has trouble finding the right focusing point when you use autofocus, don't spend too much time fiddling with the different autofocus settings. Just set the camera to manual focusing and twist the focusing ring to set focus yourself.
For movies with sound enabled: To avoid hearing the sound of the autofocus motor during the recording, which can occur with the AF-F Focus mode, use manual focusing or select the AF-S autofocus mode and lock focus before you start recording.