You haven't started the Postgres server. Some of the dmg packages for Postgres set it to run as a service on startup. But not however you did the install.
You need to init a data directory, start postgres, and then go from there.
initdb /some/directory # just do this ONCE
pg_ctl -D /some/directory start # many other options, e.g. logging, available here
psql postgres
You can set an environment variable for the data directory and you won't need the -D
flag later. You can look that up later.