psql is a terminal-based front-end to PostgreSQL. It enables you to type in queries interactively, issue them to PostgreSQL, and see the query results. Alternatively, input can be from a file.[1]

Launch Interactive session
psql -h localhost -U postgres -d somedb
connect to local database i.e.(username = postgres, database name = postgres)
sudo -u postgres psql -d postgres

Connection options:

 -h, --host=HOSTNAME database server host or socket directory
 -p, --port=PORT database server port number
 -U, --username=NAME connect as specified database user
 -W, --password force password prompt (should happen automatically)
 -e, --exit-on-error exit on error, default is to continue
 -d DBNAME some database

connect to database dbname_db with user name = postgres:

\c dbname_db postgres

change a user's password:

ALTER USER username WITH PASSWORD 'mypassword'; 

create db

CREATE DATABASE db_name;

grant privileges to user

GRANT ALL PRIVILEGES ON DATABASE db_name TO user_name;

reference:

[1]https://www.postgresql.org/docs/9.4/static/app-psql.html
[2]https://www.postgresql.org/docs/9.2/static/sql-alterrole.html