You want to record what's happening on your Ubuntu desktop on a film i.e. filming what you are doing on your computer :follow the guide.
On Ubuntu, to make a movie on what you are doing on your computer there are many programms.
I have chosen RecordMyDesktop that is probably the best offer right now : any how, it works well and simply.
WHATCH OUT for the little icon the notification area when the recording starts.
An ogv file will be generated at the end of the recording : that can take some time.
About the sound, follow Ubuntu-doc page, if you have any troubles.
If you want to record directly the sound from the output of the computer (without using any microphone), you can loop the soud output on the input, in the tab Recording of Pavu. You need to have the recording program (in our case RecorMyDesktop) to have started, and then to chose it in the list. More details on amoweb.
Happy recording ! 