How to get the Nikon D3100 to auto focus in video mode?

I just got a Nikon d3100 and love it. But in video mode it doesn't auto focus like everyone else's.
How do I set it up for auto focus while its recording?

"auto focus like everyone else's"

You are working on bad information.

It has only been recently that auto-focus in the video mode has been available and usually it is slow

Look on page 43 of your user manual to see how the auto-focus works when shooting video mode (live view)

Since shooting video usually is done using a script, manual focus is preferred in any case, usually using an LCD magnifying loupe

http://www.luminous-landscape.com/reviews/accessories/hoodloupe-3.shtml

Not all dSLRs can focus during video mode. The ones that do use slow contrast detection that would be Canon, Nikon and Pentax. Only one brand can focus, and focus fast when shooting video, and that's the dSLT cameras from Sony, the equivalent (roughly) to the D3100 would be the Sony Alpha A37.