I'm Ubuntu user for years now. I have gained a lot of knowledge on how to customize my Ubuntu box and how to make it better. This guide targets any Ubuntu user or any one is thinking about trying Ubuntu but it will be very useful for newbies.
Part 1 - Repositories and Packages
A software repository (sometimes abbreviated as a repo) is a storage location from which software packages may be retrieved and installed on a computer.
Ubuntu depend mainly on repositories to get and install applications. You should know how to add repositories and manage them.
You can add a new repository using Software Sources:
1. go to System (from the top menu) > Administration > Software Sources
Part 1 
Part 2 - Internet Tools
This part describes for Ubuntu newbies how to setup the Ubuntu box for best Internet experience. I'll cover Essential Browser extensions, Instant messaging configuration and some other tips.
As almost every one knows, Ubuntu come with Firefox as the default internet browser. But Ubuntu doesn't install main browser extensions by default. Those extensions are: Adobe flash player and Java.
To install flash, just go to Synaptic Package manager and search for flashplugin-nonfree, choose it and then click on apply.
A more simpler solution is to go to any site that have flash contents, a message will appear telling you that a plugin is needed; just click on the message and choose the Adobe plugin. Note that Ubuntu provide other (free) plugins to show flash contents but these plugins might not show all flash sites correctly.
To install Java, do the previous steps but search for sun-java6-plugin. You can either go to a website that provide Java contents and install the plugin.
Part 2