Create the file /etc/apt/sources.list.d/pgdg.list, and add a line for the repository
sudo echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -sc`-pgdg main" > /etc/apt/sources.list.d/pgdg.list
Import the repository signing key, and update the package lists
wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
sudo apt-get update