Configure your Ubuntu repositories
Configure your Ubuntu repositories to allow "restricted," "universe," and "multiverse." You can follow the Ubuntu guide for instructions on doing this.
Setup your sources.list
Setup your computer to accept software from packages.ros.org. ROS Indigo ONLY supports Saucy (13.10) and Trusty (14.04) for debian packages.
sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'
Set up your keys
sudo apt-key adv --keyserver hkp://ha.pool.sks-keyservers.net --recv-key 421C365BD9FF1F717815A3895523BAEEB01FA116
You can try the following command by adding :80 if you have gpg: keyserver timed out error due to a firewall
sudo apt-key adv --keyserver hkp://ha.pool.sks-keyservers.net:80 --recv-key 421C365BD9FF1F717815A3895523BAEEB01FA116