First update the repositories with sudo apt-get update

Install postgres: sudo apt install postgresql postgresql-contrib

Verify that it is already installed: sudo -u postgres psql -c "SELECT version();"

output:

 PostgreSQL 11.5 (Debian 11.5-1+deb10u1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit