Replying to What to know before starting filming with DSLR ?

I searched around on NS but didn't be able to answer my question.. And i thought that it should have a sticky about it.. but anyway,

I was wondering what i need to know before getting my t2i (it's in the mail right now) if i only got a gopro before..

I know that:

ISO speed: A measurement of how quickly your camera’s sensor absorbs light.

Shutter speed: A measurement of how long your camera’s shutter is open for when you take a shot.

Aperture: A measurement of how wide the shutter on your camera’s lens opens up when you take a shot.

But on what base did i adjust them ? Is there are chart or something like that that tell me how to do? and why did i need to do it ?