repository - How to add ppa repositories?

I always like to have my programs up to date but in ubuntu software center the software aren't always up to date.
I know about repositorys and i'd like to add some so how can i find the repository of certain programs in launchpad.
I'm using Ubuntu 12.04.1.

You can add a PPA location with the command

sudo add-apt-repository ppa:

For example sudo add-apt-repository ppa:gnome-desktop

You can find some help on Ubuntu Documentation