Install package
$ sudo pacman -S postgresql

Login as PostgreSQL user
$ sudo -iu postgres

Initialize database cluster
[postgres]$ initdb --locale=en_US.UTF-8 -E UTF8 -D /var/lib/postgres/data

Start and enable postgres service (as root)
[postgres]$ exit
$ sudo systemctl enable postgresql.service
$ sudo systemctl start postgresql.service

Login as PostgreSQL user and create a new user $ sudo -iu postgres
[postgres]$ createuser -d -w irargdb

Create a new database
[postgres]$ createdb argdb -O irargdb

Check connection to database
[postgres]$ createdb argdb -O irargdb

To login from shell use following command:
$ psql -U irargdb -d argdb