You shouldn't be using the superuser account for your normal database work. That is as if you were using root for everything in Linux.
You need to create a regular user with the privileges to create or modify tables in your database. This can be done by granting the user all privileges on the database (which is not the same as making that user a superuser) or make that user the owner of that database.
As documented in the manual psql tries to connect to a database with the name of the current Linux user and with a database user with the name of the current Linux user. So if you want to keep things simple create a user with your regular Linux user's name and an database that is owned by that user:
create user rob password 'somepassword';
create database rob owner = rob;
Assuming your Linux user is rob, then all you need to do is:
psql
and you are connected to a database where you can create and manage tables.
Depending on how you installed Postgres, you might need to adjust pg_hba.conf to allow rob to log in directly.
Again: please do NOT use the superuser account for your normal work.
psql -U postgres?