Go to Terminal ( Applications -> Accessories -> Terminal ) and type the following command.
sudo aptitude install ubuntu-restricted-extras
You will be prompted to enter the password of your 'root' account. Enter that and confirm the Installation ( you will be asked to press yes/no -- hit a 'y' ).
Through this pack you will be able to get codecs to play MP3 files and other GStreamer Plugins, Microsoft Fonts, JRE and Flash Plugin and a host of other features.