First you have to create a movie in digital format suitable for computers.
Digital format means saved to the cameras memory and not film used in older cameras.
Then you have to transfer the movie from your camera to the computer through a special cable.
When it's on the computer you can use an editing program (if you have one) to change the movie to get it looking how you want it to.
To let others see it online, you have to store it (upload it) to your own WebSite or to a hosting service such as MySpace, YouTube or Google Videos.