Creating user, database and adding access on PostgreSQL

sudo -u postgres psql
postgres=# create database mydb;
postgres=# create user myuser with encrypted password 'mypass';
postgres=# grant all privileges on database mydb to myuser;

As the default configuration of Postgres is, a user called postgres is made on and the user postgres has full superadmin access to entire PostgreSQL instance running on your OS.

$ sudo -u postgres psql

Creating user

$ sudo -u postgres createuser <username>

Creating Database

$ sudo -u postgres createdb <dbname>

Giving the user a password

$ sudo -u postgres psql
psql=# alter user <username> with encrypted password '<password>';

Granting privileges on database

psql=# grant all privileges on database <dbname> to <username> ;

postgres-# \l
                             List of databases
   Name    |  Owner   | Encoding | Collate | Ctype |   Access privileges   
-----------+----------+----------+---------+-------+-----------------------
 postgres  | postgres | UTF8     | C       | C     | 
 template0 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
 template1 | postgres | UTF8     | C       | C     | =c/postgres          +
           |          |          |         |       | postgres=CTc/postgres
 testdb    | postgres | UTF8     | C       | C     | 
(4 rows)

postgres-#