First you need to create a user. You can create a username toolel and password helloworld123 by
CREATE USER toolel WITH PASSWORD 'helloworld123';
After this we can create an database that the user can use:
CREATE DATABASE tooleldb;
But this wont be so useful unless the user gets privileges on the database:
GRANT ALL PRIVILEGES ON DATABASE tooleldb to toolel;
This grants all privileges for the database, but PostgreSQL have many more ways to give certain rights and not others.