To understand PostgreSQL, you need to see it in use. An empty database is like a ghost town without houses.
For now, we will assume that you already have a database. There are over a thousand books on how to design your own database from nothing. So here we aim to help people who already have access to a PostgreSQL database, but are still learning to use the PostgreSQL database management system.
The best way to start is by asking some simple questions to orient yourself and begin the process of understanding. Incidentally, these are also questions you'll need to answer if you ask someone else for help.