To login
sudo su - postgres
psql
\l #lists databases
\connect database_name #to connect to that database
\dt #lists tables
To quit
\q
exit #to get out of the postgres user
To drop tables from Postgres, documentation here
You generally need to do this when there is some problem with migrations, and you need to drop a table so a migration can build it.
DROP TABLE table_name CASCADE; #to drop a table, cascades and deletes dependencies
To add tables to Postgres, documentation here
In general, you shouldn't need to do this. This should always be done with migrations. Remember, you can write SQL migrations.