How to configure apt-get after you had ubuntu installed.

how-to-configure-apt-get.md

Proxy settings

Touch the proxy configuration file if needed

sudo vi /etc/apt/apt.conf.d/80proxy

Add the following configuration:

Acquire::http::proxy "http://<proxy>:<port>/";
Acquire::ftp::proxy "ftp://<proxy>:<port>/";
Acquire::https::proxy "https://<proxy>:<port>/";

And then add some source from here, those changes should be applied in file /etc/apt/sources.list

deb http://archive.ubuntu.com/ubuntu/ raring main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ raring-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ raring-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ raring-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ raring-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ raring main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ raring-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ raring-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ raring-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ raring-backports main restricted universe multiverse

After all of those steps, perform the update to let apt-get know where to fetch the content afterward.

sudo apt-get update