How to control video settings on a DSLR? (nikon)?

I shoot videos with my Nikon D5000, and i was wondering why my video changes like the white balance and sometimes it flickers bright to dark. And it ruins my videos. How can i control that and leave it as one setting? How can i control other settings such as shutter speed and stuff during video so that i can manually change it and not automatic?

ps. Can someone explain to me what shutter speed and white balance does to video?

Read the manual. Don't expect much. The dSLR wasn't really meant to do video so up to now it's just a feature.

To control White Balance, refer to page 96 of your manual. Essentially take it out of Auto mode.
To control shutter speed and stuff, take the camera out of Auto mode and put it in one of the other modes such as Shutter priority as explained on page 5 of your manual.
Everything is about exposure, teach yourself or take a photography course and learn about exposure. It's essentially the same between still and movie photography.
You can download your manual from the Nikon site.