By default, Ubuntu does not enable the Universe and Multiverse repositories. But they include some important programs and codecs, so it is highly recommended to activate them. There may also be non-Ubuntu sources that you might wish to add to your Ubuntu machine.
The repositories (of apt) are stored in a file called /etc/apt/sources.list
First you should create a backup of your existing sources.list file:
sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
Next you edit the sources.list file with an editor like GEdit.
sudo gedit /etc/apt/sources.list
There is an incode documentation. Uncomment everything you need, especially the universe and multiverse repositories.
Save the file and run
#sudo apt-get update