To get the correct package name, use:
apt-cache search package
Then install it (as root) using:
apt-get install package-name
You can also use one of the graphical package managers such as synaptic.
You may need to edit /etc/apt/sources.list and then run "apt-get update" if the package you want is not found. There is a stick at the top of this forum with plenty of repositories.
I hope this helps