Not sure what you are wanting to do? You can use Quicktime Pro to do a screen recording, or you can use the built in iSight camera for imessaging, etc, but a "video"usually means a movie of some sort and the iMac is not a video camera - it does come with iMovie though which is a great video editing app.
So, please explain exactly what you are wanting to do since I might have misunderstood.....
"How to" instructions are provided in the Help menu commands for any of the video capable apps that came with you Mac, including those that babowa has listed for you. There are also web-based tutorials and help articles for several of these apps. Some examples: