How to fix error when creating user in PostgreSQL?

Question

PostgreSQL

Trying to install PostgreSQL, ran the following commands:
1) sudo apt-get install libpq-dev python-dev
2) sudo apt-get install postgresql postgresql-contrib
3) pip install psycopg2==2.7.4
4) su postgres and here after after I entered the password it says (su: Authentication failed)
On the Internet, all I found was to change the password using the sudo -s command