1. ROS Installation

Setup your sources.list
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 -

Installation
Make sure your Debian package index is up-to-date
sudo apt update
Desktop-Full Install: (Recommended) : ROS, rqt, rviz, robot-generic libraries, 2D/3D simulators and 2D/3D perception
sudo apt install ros-melodic-desktop-full
Desktop Install: ROS, rqt, rviz, and robot-generic libraries
sudo apt install ros-melodic-desktop-full
ROS-Base: (Bare Bones) ROS package, build, and communication libraries. No GUI tools.
sudo apt install ros-melodic-ros-base

Environment setup
It's convenient if the ROS environment variables are automatically added to your bash session every time a new shell is launched:
echo "source /opt/ros/melodic/setup.bash" >> ~/.bashrc
source ~/.bashrc

Dependencies for building packages
sudo apt install python-rosdep python-rosinstall python-rosinstall-generator python-wstool build-essential
sudo apt install python-rosdep
sudo rosdep init
rosdep update