First, make sure your Debian package index is up-to-date:
sudo apt update
Now pick how much of ROS you would like to install.
Desktop-Full Install: (Recommended) : Everything in Desktop plus 2D/3D simulators and 2D/3D perception packages
sudo apt install ros-noetic-desktop-full
or click here
Desktop Install: Everything in ROS-Base plus tools like rqt and rviz
sudo apt install ros-noetic-desktop
or click here
ROS-Base: (Bare Bones) ROS packaging, build, and communication libraries. No GUI tools.
sudo apt install ros-noetic-ros-base
or click here
There are even more packages available in ROS. You can always install a specific package directly.
sudo apt install ros-noetic-PACKAGE
e.g.sudo apt install ros-noetic-slam-gmapping
To find available packages, see ROS Index or use:
apt search ros-noetic