It will not continually auto-focus like a dedicated video camera. It should auto-focus if you hit the focus button (shutter button or, if you have it set up for BBF, then the back button). Unfortunately, the AF motor is ungodly loud when you watch the playback, so all of the how-to videos on Youtube suggest manual focus (or an expensive focus puller), or an external microphone. I bought the Nikon external microphone that mounts on the hot-shoe, and you can still hear the auto-focus.
These German guys have some great videos on how to use and set your DSLR for video. The videos are in English:
https://www.youtube.com/user/FenchelJanisch2