Ubuntu Sources List Generator is a website where you can generate a sources.list file with a lot of useful repositories.
Simply select your Country and your Ubuntu Release (version) on top of the page and you will be able to select many repositories to add to the sources.list. Once you have selected all the repositories you want, click the “Generate” button and you will then be able to copy the repositories into your sources.list file. To do this open a Terminal (Applications->Accessories->Terminal) and type:
gksu gedit /etc/apt/sources.list
Then copy and paste the generated file to your sources.list
If you need to add GPG Keys open a Terminal (Applications->Accessories->Terminal) and type:
sudo apt-key adv –recv-keys –keyserver keyserver.ubuntu.com YOURGPGKEYHERE