# install **wget** if not already installed:
sudo apt install -y wget

# import the repository signing key:
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -

# add repository contents to your system:
echo "deb https://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" | sudo tee  /etc/apt/sources.list.d/pgdg.list

# install and launch the postgresql service:
sudo apt update
sudo apt -y install postgresql-11
sudo service postgresql start