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
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
Before you can use many ROS tools, you will need to initialize rosdep. rosdep enables you to easily install system dependencies for source you want to compile and is required to run some core components in ROS.
sudo apt install python-rosdep python-rosinstall python-rosinstall-generator python-wstool build-essential
sudo apt install python-rosdep
Initialize rosdep
sudo rosdep init
rosdep update