In a nutshell
edit the file /etc/apt/sources.list
You can edit it from the command line ( http://help.ubuntu.com/community/Repositories/CommandLine ) or use the GUI from the Ubuntu desktop: System –> Administration –> Software Properties and add to the repositories by adding a channel (the add button). You can also add new repositories there as well.
For a very well documented HOWTO: visit http://help.ubuntu.com/community/Repositories/Ubuntu