Ubuntu uses a wonderful point-and-click tool called Synaptic Package Manager
. If you know the name of the application you want to install, you can open Synaptic and look for it on the list that appears, and click it and choose Mark for Installation
. You can even narrow your search by category on the left side of the screen.
After you have checked the applications that you want, and they have been marked for installation, click on the Apply button at the top, and watch the magic happen! It grabs the application from the repositories and installs it for you, and tells you when it's done.
The command line is a faster way to do it, if you know the name of the application. Let's say it's Abiword you want. You can just open a terminal and type
sudo apt-get install abiword
If you tell us what you want to install, we can help you better.