The gold standard for testing databases are made by the Transaction Processing Council (TPC).
They have created a bunch of tests over the years allowing vendors to compete on different typical data loads, such as transaction processing or decision support. Their tests come with a schema and a tool that generates data for any data volume you would like to test. The queries to be used in their benchmarks are also tool generated.
You can go the hard route and download one or more of their tests (TPC-C, TPC-H, TPC-DS) and figure out how to make it work with Postgres.
Here is a page that describes how to use the venerable TPC-H benchmark: TPC-H - PostgreSQL wiki
Or you can take a shortcut and simply use the built-in benchmark that ships with PostgreSQL (pgbench). This one, to not run afoul of the TPC, is only loosely based on TPC-C.