General instructions for installing ROS can be found here.
if installing on a desktop PC, install ROS Desktop-Full, on a companion computer ROS-Base is enough
add ROS tool to your shell as stated in ROS wiki
do not use root as the default user
MAVROS is compatible with all recent versions of ROS including Kinetic, Melodic and Noetic.
Instructions for installing MAVROS can be found here but in short involve running the following command.
sudo apt-get install ros-noetic-mavros ros-noetic-mavros-extras wget https://raw.githubusercontent.com/mavlink/mavros/master/mavros/scripts/install_geographiclib_datasets.sh chmod a+x install_geographiclib_datasets.sh ./install_geographiclib_datasets.sh
For ease of use on a desktop computer, please also install RQT
sudo apt-get install ros-noetic-rqt ros-noetic-rqt-common-plugins ros-noetic-rqt-robot-plugins