In ubuntu:
system > administration > synaptic package manager
http://ubuntuguide.org/
.. for a good rundown. It is for 5.04, but the information is basically OK.
You want to know about installing extra repositories.
applications > accessories > terminal
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
sudo gedit /etc/apt/sources.list
uncomment all the repositories which have been commented out. Then:
sudo apt-get update
... or use synaptic.
You should be able to find pretty much everything you want here.
For firefox plugins (macromedia, javascript etc) open firefox and visit the mozilla webpage (it's in your bookmarks). There are also extensions and themes for firefox there.
Generally, installing and updating software in Ubuntu is actually
easier than in windows. Also quicker.
Also, take a look at the second menue down (My LQ) over there >>>--->
... from there you can modify stuff in your profile (click on "Edit Profile"). Go through the list - the thing I'd like you to list is your distro (Ubuntu) and your location. This helps experienced members answer your posts. Thanks.