Important note: Most of the howtos in this forum need the Universe and Multiverse repositories to be enabled.

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.

If you prefer to do it graphically with the apt-frontend Synaptic, there is a detailled manual with screenshots at https://wiki.ubuntu.com/AddingRepositoriesHowto.