Installing a package from the Ubuntu repositories is easy. There are three ways to do it.
You can use the Ubuntu Software Center. Open it, search for the package name and install it.
You can use the Synaptic program. You may have to install it, if it is not already there. Open it, search the package name, mark it for installation and choose Apply.
If you like the terminal, use the apt-get program. For example, to install a package named Foo:
$ sudo apt-get install Foo
Tried with: Ubuntu 12.04.1 LTS