The following steps work for a fresh install of postgres 9.4 on Ubuntu 14.04
By default, postgres creates a user named 'postgres'. We log in as that user, and give that user a password.
$ sudo -u postgres psql
Enter password: topseckret
to exit postgres.
Then we connect as 'postgres'. The -h localhost part is important: it tells the psql client that we wish to connect using a TCP connection (which is configured to use password authentication), and not by a PEER connection (which does not care about the password).
$ psql -U postgres -h localhost