In order to install

1. Retrieve the current list of packages from all servers

    $ sudo apt-get update 
   
   
2. Do a keyword search

    $ apt-cache search <keywords>
   
   
3. Get more information

    $ apt-cache show <program>
   
   
4. Install a package

    $ sudo apt-get install <program>
   
   
5. Remove a program

    $ sudo apt-get remove <program>
    $ sudo apt-get remove --purge <program> 
   
6. Upgrade your system

    $ sudo apt-get upgrade
   
   
7. Perform a full upgrade

    $ sudo apt-get dist-upgrade
   
   
8. Clean up

    $ sudo apt-get clean
   
   
9. Clean smarter

    $ sudo apt-get autoclean
   
   

Add the Universe repository

Uncomment the following lines in /etc/apt/sources.list

 $ sudo vi /etc/apt/sources.list
## Uncomment the following two lines to add software from the 'universe'
## repository.
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## universe WILL NOT receive any review or updates from the Ubuntu security
## team.
 deb http://se.archive.ubuntu.com/ubuntu/ edgy universe multiverse
 deb-src http://se.archive.ubuntu.com/ubuntu/ edgy universe multiverse

multiverse for mplayer

See also AptGetHowto