In Linux, you will need to use the command line interface. Fortunately, in Ubuntu, and other "out of the box" distros, you will not need it. But in minimalist distros, and if you install a base Ubuntu install or similar, you will need it. If you want to find out detailed information about a command, run
man [command]

Package managers

If you want to know how to use pacman, Arch Linux's package manager, see this page. If you are using Ubuntu, Debian or derivatives, you can use a program called "Synaptic Package Manager" which is a GUI frontend for APT. For Kubuntu it is called "Adept Manager".

cd

cd means "change the working directory" and basically means move to a different folder. If you want to change the directory to /usr, I would use
cd /usr
Let's say I'm in my home folder, and I want to change the directory to Pictures/Wallpapers. Instead of typing
cd /home/username/Pictures/Wallpapers
I can just type
cd Pictures/Wallpapers
I you want to change the directory to the one above it, do
cd ..

ls

ls means "list directory contents", which means list all files and folders contained in the specified folder. If you wanted to see what files and folders are in the directory you are in, simply type
ls
If you want to find out what files are in /usr/share, you would run
ls /usr/share
If you want to show hidden files (files with a "." before them) run
ls [folder] -a

cat

Concatenate files and print on the standard output. What this means is to see what a file contains. If I wanted to see what my /etc/rc.conf had in it, I would run
cat /etc/rc.conf

man

Format and display the on-line manual pages. This means show manual pages in a readable way. If you wanted to find out how to use ls in more depth, you would run
man ls
You can quit with "Q" and use the cursor keys to go up and down, and the space bar to go down a page.

Tip!
You can use the tab button to automatically complete the directory name. Let's say I want to change the directory to Pictures/Some really long name. I could type in
Pictures/Some
and press tab. It will automatically complete it for you and it will end up like this:
Pictures/Some\ really\ long\ name
Also notice the backwards slashes. You must put them there if there are spaces in the filename. You can put these before the command. For example:
cd Pictures/Some
then press tab to get
cd Pictures/Some\ really\ long\ name