Since I install Ubuntu, I can't listent to mp3 music or watch video under .avi format. So I have to download pluggin for those file format. But there is so many files in the library and I am confuse on what to look for. Is there a way I can identify those files and download accordingly? By the way I am using Wifi area with a notebook for the download as my home do not have an internet yet.

Try to play your MP3 files with Totem. It should automatically suggest installing the plugin and install it for you. If this isn't happening then do the following:

a) Under the menu: System -> Administration, find the program labeled "Software Sources" and enable "Software restricted by copyright or legal issues (multiverse)"

b) Under the same menu look up "Synaptic" and run it. Run a search on "Ubuntu-Restricted-Extras" and install it. This will enable you to play MP3's (among other things as many more codecs will be installed) and also give you some extra packages (which are sometimes useful) like Microsoft Fonts. This should be all you need.

By the way, all the codecs that are run under Ubuntu start with the name "gstreamer". If you run a search on them in Synaptic you'll have an idea of what's available. They have names like: gstreamer0.10-ffmpeg (AVI playback and another 90 formats).

if you install vlc you can pretty much watch anything but to use other medial players you have to install the restricted extras

I got no problem with Debian and Linux Mint. But will try again on Ubuntu. Thanks!