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.
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 install curl # if you haven't already installed curl curl -s https://raw.githubusercontent.com/ros/rosdistro/master/ros.asc | sudo apt-key add -