Tips on shooting video with DSLR?

I have been into photography for a few years now and spent most of those years using a Nikon D80. However, I recently upgraded to a Nikon D7000 after getting a great deal from a friend.

I have never messed around with the video option on the camera, but a recent uni assignment is forcing me to get started on that. I'm required to shoot a short commercial video tomorrow and I've been testing the camera out all day.

I'm having difficulty in getting the video tack sharp like some amateur videos I've seen on Youtube.
Youtube vloggers such as Charlieissocoollike and Kev Jumba seem to have really sharp videos and I know for a fact they're shooting with a DSLR too. What do you think? Is it due to lighting, lens, technique or anything else?

My camera is paired with a kit lens Nikkor 18-135mm by the way. Also, I messed around with the rotation dial and it seems to switch some numbers around on the display. Does the same concept of shutter speed apply for video? I can't imagine how… However, I do think that we can adjust the aperature to adjust the brightness of the video.Am I right in assuming this?

As for audio quality, I know that I shouldn't be expecting the best with this built in microphone… Infact, it was pretty horrible. My videos were all pretty much ruined by the sound of the wind and external noises. I expect to get a lot of these sort of assignments in the future, so I'm planning on investing on a good external microphone with a reasonable price. Can you guys give me some suggestions on that?

Any help is appreciated.

Shutter speed does play a part in video. Basically there's a framing rate of 25 or 30fps, so an equivalent shutter speed of around 1/30s, but nearer 1/25s. If you use a shorter shutter speed, say 1/125s or 1/250s this has the effect of freezing motion on each frame. You can alter shutter speed and aperture depending on the light level. See the link.

As for focus, most DSLRs don't have continuous AF so you either have to keep focussing manually or half press the shutter button to AF.

Can't comment on microphones. You can shield the built-in microphone ports to some extent to protect from wind noise. I hold my hand over the microphone on my camcorder to shield it and it gets rid of a lot of noise.

The D7000 with a good lens is tack sharp. I have shot plenty of jobs on it. You can see this video which I shot on the D7000 with Zeiss and Nikon lenses. Https://vimeo.com/35868211

Make sure you are set at 1080 24. Focus carefully, preferably with an external monitor or a or an EVF.

For audio you should record on a digital recorder and get a good mic. This is not a cheap area. You can get a nice recorder from Zoom or Tascam and I use Sennheiser mics. It will be extremely difficult to get good quality audio on the camera.