How to set auto focus for video mode on a nikon d3100 camera?

I just got a nikon d3100 and i can't seem to get the auto focus to work. Its a used camera and it takes great pics and auto focuses for pics but not for video and i can't figure out why. Anyone know the settings for auto focus video mode?

Because this camera doesn't have auto focus in video. If I were you I'd learn how to manually focus your lens.

This is why you do your research before you buy a camera, and don't think that getting a Nikon or Canon makes it a safe bet. So I guess you found out that your camera will not AF in video mode.

For the best video performance in a dSLR type camera, you should have purchased a Sony Alpha, nobody does video better. For video not only do you get auto focus, you get fast phase detect auto focus, and for manual focusing if that's what you need, you get focus peaking to make it a breeze, Canon and Nikon don't have either feature, at any price, in any model.

The A57 and A58 Sony Alphas are in a similar price range as the D3100 and D3200.